Pattern forming method using mask

ABSTRACT

A mask includes a transparent layer which is transparent with respect to a light which is used for an exposure, and a mask pattern layer which is formed on the transparent layer. At least a portion of the mask patterns layer is made up solely of a phase shift layer for transmitting the light, so that a phase shift occurs between a phase of the light transmitted through the phase shift layer and a phase of the light transmitted through a portion of the mask having no phase shift layer.

This application is a division of application Ser. No. 08/054,608, filedApr. 30, 1993, now U.S. Pat. No. 5,489,509, which is a continuation ofapplication Ser. No. 07/516,347, filed Apr. 27, 1990, now abandoned.

BACKGROUND OF THE INVENTION

The present invention generally relates to masks, mask producing methodsand pattern forming methods using masks, and more particularly to a maskwhich uses a phase shift of light, a mask producing method for producingsuch a mask and a pattern forming method which uses such a mask.

When forming patterns of elements, circuits and the like on asemiconductor wafer in a production process of a semiconductor device,it is normal to employ a pattern transfer exposure which uses anultraviolet light.

The pattern which is to be transferred onto the wafer is formeddepending on the existence of a metal thin film which is provided on aglass substrate, where the metal thin film is opaque with respect to thelight and the glass substrate is transparent with respect to the light.The pattern on the glass substrate to be transferred onto the wafer iscalled a mask when this pattern is identical to a chip pattern which isactually transferred onto the wafer. On the other hand, the pattern onthe glass substrate is called an enlarged mask or a reticle when thispattern is enlarged compared to the chip pattern which is actuallytransferred onto the wafer. When using the mask, the pattern istransferred onto a resist layer on the wafer and exposed using aparallel ray. On the other hand, when using the reticle, the pattern istransferred onto the resist layer on the wafer and exposed using areduction lens system which projects a reduced pattern on the resistlayer.

In order to improve the resolution of the pattern which is transferredespecially when the integration density of the integrated circuit islarge and the pattern is fine, it is necessary to improve the contrastat an edge portion of the region which is exposed.

FIG. 1 shows an example of a conventional mask which is made up of anopaque layer having a predetermined pattern and a transparent substrate.FIG. 2 shows an example of an optical system for forming a pattern onthe wafer using the mask shown in FIG. 1.

In FIG. 1, a mask 450 comprises a transparent substrate 452 and anopaque layer 451 which is made of a material such as chromium (Cr). Theopaque layer 451 is formed to a predetermined pattern by lithography andetching processes.

In FIG. 2, a light C which is emitted by an exposure apparatus (notshown) illuminates the mask 450. The light C cannot be transmittedthrough the opaque layer 451 but is transmitted through a portion of thetransparent substrate 452 not provided with the opaque layer 451. Thetransmitted light passes through an imaging lens system 453 and exposesa resist material 455 which is coated on a wafer 454. For example, aresist OFPR manufactured by Tokyo Ooka Kogyo K. K. of Japan may be usedas the resist material 455. As a result, a pattern identical to thepattern of the mask 450 is formed on the wafer 454 by an etchingprocess.

When forming the pattern using an optical lens system, the exposure onthe wafer 454 is made based solely on data related to the contrast whichis determined by the existence of the opaque layer 451 on thetransparent substrate 452. For this reason, there is a physicalresolution limit to the pattern formation due to the wavelength of thelight which is obtained via the optical lens system, and it is difficultto form a fine pattern depending on the wavelength of the light used forthe exposure.

Conventionally, the photolithography process uses a reticle which ismade by forming an opaque layer on a transparent substrate andpatterning the opaque layer. For example, the opaque layer is made of Crand the transparent substrate is made of a transparent material such asglass and quartz. FIGS. 3A through 3D are diagrams for explaining such aconventional pattern forming method.

In FIG. 3A, the optical system includes a light source 461, anillumination lens 462, and an imaging lens system 465. A reticle 464 isarranged between the illumination lens 462 and the imaging lens system465. The light source 461 is made of a mercury lamp, an excimer laser orthe like and is provided with a filter for emitting i-ray or g-ray. Thelight 463 from the light source 461 illuminates the reticle 464 via theillumination lens 462. For example, the partial coherency σ of theillumination lens 462 is 0.5. The reticle comprises a transparentsubstrate 468 and an opaque pattern 469 which is formed on thetransparent substrate 468. For example, the transparent substrate 468 ismade of glass and the opaque pattern 469 is made of Cr. The opaquepattern 469 on the reticle 464 is imaged on a photoresist layer 467which is formed on a semiconductor substrate 466 via the imaging lenssystem 465. For example, the imaging lens system 465 has a numericalaperture (NA) of 0.50.

According to this pattern forming method, the resolution can bedescribed by K1·λ/NA, where K1 denotes a process coefficient which isnormally 0.6 to 0.8 and λ denotes the wavelength of the light. Thewavelength λ of the light 463 is approximately 365 nm in the case of thei-ray which is emitted using the mercury lamp and is 248 nm or 198 nmwhen the excimer laser is used. The numerical aperture NA differsdepending on the imaging lens system 465 which is used but isapproximately 0.5, for example. In order to improve the resolution, itis necessary to set K1 or λ to a small value and set NA to a largevalue. However, the values of K1 and NA cannot be set freely. Inaddition, the value of λ is restricted by the light source 461 and theoptical system used. The resolution is determined when the wavelength λof the light used for the exposure, the numerical aperture NA and theprocess coefficient K1 are determined, and it is impossible to image apattern finer than the resolution.

The light 463 emitted from the light source 461 illuminates the entiresurface of the reticle 464, and the portion of the light 463 whichilluminates the opaque pattern 469 is stopped by the opaque pattern 469.For this reason, only the portion of the light 463 which illuminates theportion of the reticle 464 not provided with the opaque pattern 469 istransmitted through the reticle 464 and is imaged on the photoresistlayer 467 via the imaging lens system 465. FIG. 3B shows an electricalvector E of the light transmitted through the reticle 464, and FIG. 3Cshows a light intensity P of the light transmitted through the reticle464. A pattern having a light intensity distribution proportional to thesquare of the amplitude of the light illuminating the photoresist layer467 is formed on the photoresist layer 467 and the photoresist layer 467is selectively exposed.

FIG. 3D shows a portion of the reticle (opaque mask) 464 on an enlargedscale. A minimum width W of the pattern which can be exposed isrestricted by the resolution which is determined by the imaging lenssystem 465.

Next, a description will be given of the light intensity distributionfor cases where the pattern which is exposed is a line pattern which isfiner than the resolution, by referring to FIGS. 4A through 4D. In FIGS.4A through 4D, the wavelength λ of the light used is 365 nm, thenumerical aperture NA is 0.50 and the partial coherency σ of the lightis approximately 0.5.

FIG. 4A shows the light intensity distribution for the case where theline pattern which is imaged has a width of 0.35 μm. The light intensityapproaches zero approximately at a center position (0.0) and graduallyrises on both sides of the center position (0.0). The line width isapproximately 1.0 μm or greater at a position where the light intensityis a maximum. When the photoresist layer is developed using a positionwhere the light intensity is approximately 0.2 as a developingthreshold, it is possible to develop a pattern which has a designedwidth in the order of 0.35 μm.

FIG. 4B shows the light intensity distribution for the case where theline pattern which is imaged has a width of 0.30 μm. A minimum of thelight intensity at the center position (0.0) is increased compared tothe light intensity distribution shown in FIG. 4A. The width of thelight intensity distribution itself shown in FIG. 4B is not muchdifferent from that of FIG. 4A.

FIGS. 4C and 4D respectively show the light intensity distributions forthe cases where the line pattern which are imaged have widths of 0.25 μmand 0.20 μm. In each of FIGS. 4C and 4D, the minimum of the lightintensity at the center position (0.0) is increased compared to thelight intensity distribution shown in FIG. 4A, similarly to the caseshown in FIG. 4B. The width of the light intensity distribution itselfshown in each of FIGS. 4C and 4D is not much different from that of FIG.4A. In other words, even when the pattern width is reduced exceeding theresolution, the pattern width of the light intensity distribution whichis obtained does not decrease and the minimum of the light intensity atthe center position (0.0) increases. In these cases, it is impossible toreduce the line width which is exposed, and the black level is exposedas a gray level. For these reasons, it is impossible to form an imagewhich is finer than the resolution.

On the other hand, a method of shifting the phase of the light which istransmitted through the mask and exposed on the wafer by 180° dependingon the patterns of the mask is proposed in Marc D. Levenson, "ImprovingResolution in Photolithography with a Phase-Shifting Mask", IEEETRANSACTIONS ON ELECTRON DEVICES, Vol. ED-29, No. 12, December 1982.According to this proposed method, the interference between the patternsis eliminated so as to improve the contrast on the wafer and improve theresolution of the exposure apparatus.

However, it is difficult to apply this proposed method to masks andreticles having fine patterns, and there is a problem in that it istroublesome to generate pattern data peculiar to the phase shiftpattern. For this reason, there is a demand to realize a phase shiftpattern which is easily applicable to masks and reticles having finepatterns and does not substantially increase the number of processessuch as the generation of the pattern data.

The phase shift pattern of the phase-shifting mask which is usedaccording to this proposed method is formed as follows. First, anauxiliary pattern is formed in a vicinity of a design pattern (whitepattern) which is to be transferred onto the wafer, where the auxiliarypattern has a width smaller than that of the design pattern. Second, aphase shifter is formed on the auxiliary pattern. The phase shifter isan organic pattern made of a resist or the like which is formed bycoating, exposure and developing processes, or an inorganic patternwhich is formed by chemical vapor deposition and lithography processes.For example, a description will now be given of a method of forming aphase-shifting mask which uses a negative resist pattern as the phaseshifter, by referring to FIGS. 5A through 5D.

In FIG. 5A, an opaque layer 552 is formed on a glass substrate 551. Anaperture pattern, that is, a design pattern 553 comprising atransmission region, and fine patterns 554A and 554B are formed in theopaque layer 552 by a lithography using an electron beam exposure. Forexample, the design pattern 553 has a width in the order of 1.5 μm, andthe fine aperture patterns 554a and 554B have a width in the order of0.5 μm. The fine aperture patterns 554A and 554B are formed as auxiliarypatterns in a region neigboring the design pattern and separated by adistance in the order of 0.5 μm, for example.

Next, as shown in FIG. 5B, a transparent conductor layer 555 forpreventing charge up at the time of the electron beam exposure is formedon the surface of the glass substrate 551 including the inner sides ofthe aperture patterns 553, 554A and 554B.

Then, as shown in FIG. 5C, a negative resist layer 656 is formed on theglass substrate 551 to a thickness such that the phase of the lightwhich is transmitted through the negative resist layer 656 is shifted by180°. A prebaking process is made if needed, and a phase shift patternis formed on the fine patterns 554A and 554B by the electron beamexposure.

A thickness D of the negative resist layer 656 can be obtained from thefollowing formula (1), where λ denotes the wavelength of the light usedfor the exposure and n denotes a refractive index of the shiftermaterial.

    D=λ/2(n-1)                                          (1)

When using the i-ray having the wavelength of 365 nm for the exposure,the refractive index n of the negative resist layer 656 is approximately1.6 and the thickness D becomes approximately 304 μm.

Next, as shown in FIG. 5D, a developing process is made to selectivelyform phase shift patterns 556A and 556B on the fine patterns 554A and554B, respectively. The phase shift patterns 556A and 556B is made up ofthe negative resist layer 656 which has the thickness D.

FIG. 6 shows a phase profile corresponding to the pattern position ofthe i-ray transmitted through the phase-shifting mask shown in FIG. 5Dwhen the i-ray is used for the exposure.

On the other hand, when a positive resist is used, a phase shift pattern557 which is made up of the positive resist is selectively formed on thedesign pattern 535 as shown in FIG. 7 by processes similar to thosedescribed in conjunction with FIGS. 5A through 5D. In FIG. 7, thoseparts which are essentially the same as those corresponding parts inFIGS. 5A through 5D are designated by the same reference numerals, and adescription thereof will be omitted.

FIG. 8 shows a phase profile corresponding to the pattern position ofthe i-ray transmitted through the phase-shifting mask shown in FIG. 7when the i-ray is used for the exposure.

According to the phase-shifting mask shown in FIGS. 5D and 7, the phaseof the i-ray (i_(a) and i_(c)) which is transmitted through the designpattern 553 and the phase of the i-ray (i_(b) and i_(d)) which istransmitted through the fine patterns (auxiliary patterns) 554A and 554Bdiffer by 180° as may be seen from FIGS. 6 and 8. For this reason, thei-ray (i_(a) and i_(c)) which is scattered in the horizontal directionfrom the region immediately below the design pattern is cancelled by thei-ray (i_(b) and i_(d)) which is scattered in the horizontal directionfrom the region immediately below the auxiliary pattern, and thecontrast at the ends of the exposure region is improved therebyimproving the resolution.

The aperture width of the auxiliary pattern is made narrow to such anextent that the light used for the standard exposure is insufficient toexpose the resist layer to the bottom portion thereof. Hence, theauxiliary pattern will not be transferred onto the wafer when theexposure is made using the phase-shifting mask.

For example, the methods described in conjunction with FIGS. 5 through 8are proposed in Japanese Laid-Open Patent Applications No. 61-292643,No. 62-67514 and No. 62-18946.

However, the conventional masks suffer from the following disadvantages.

First, in the case of the mask which uses no phase shift, it isdifficult to form a pattern which is narrower than the wavelength of thelight due to the physical resolution limit of the optical system. Whenan attempt is made to realize a narrow line width, it is necessary tomake a structural modification such as reducing the wavelength of thelight and increasing the numerical aperture. Accordingly, it isimpossible to form the fine patterns which are required in the futureintegrated circuits using the optical method.

Second, in the case of the mask which uses the phase shift, the patternformation can only be applied to a pattern such as the so-calledline-and-space pattern which has regularity, and the pattern formationcannot be applied to the production of integrated circuits havingvarious patterns. In addition, it is impossible to form a fine patternbecause an opaque layer must always be provided.

Third, in the case of the mask which uses the phase shift, the auxiliarypattern must be patterned by the exposure technique to a degree finerthat the design pattern. For this reason, the fineness of the designpattern must be restricted in order that the resolution limit of theauxiliary pattern is not exceeded.

Fourth, in the case of the mask which uses the phase shift, the numberof steps required to generate the pattern data is large because it isnecessary to generate in addition to the data related to the designpattern the pattern data related to the auxiliary pattern, the patterndata related to the phase shift and the like.

Fifth, in the case of the mask which uses the phase shift, it isdifficult to control the quality and thickness of the phase shifterwhich affects the refractive index when an organic material such as aresist is used for the phase shifter. For this reason, it is difficultto form a uniform phase shift pattern which has an accurate phase shiftquantity.

Sixth, in the case of the mask which uses the phase shift, the phaseshifter is made of a material different from the glass substrate. Thus,a reflection occurs at the boundary of the phase shifter and the glasssubstrate thereby deteriorating the exposure efficiency.

SUMMARY OF THE INVENTION

Accordingly, it is a general object of the present invention to providea novel and useful mask, a mask producing method and a pattern formingmethod in which the problems described above are eliminated.

Another and more specific object of the present invention is to providea mask comprising a transparent layer which is transparent with respectto a light which is used for an exposure, and a mask pattern layer whichis formed on the transparent layer. At least a portion of the maskpattern layer is made up solely of a phase shift layer for transmittingthe light, so that a phase shift occurs between a phase of the lighttransmitted through the phase shift layer and a phase of the lighttransmitted through a portion of the mask having no phase shift layer.According to the mask of the present invention, it is possible to formnarrow patterns having width which exceeds the conventional resolutionlimit by effectively utilizing the phase shift layer.

Still another object of the present invention is to provide a mask whichhas a mask pattern comprising a transparent layer which is transparentwith respect to a light which is used for an exposure, an opaque layerwhich is formed on the transparent layer, and a phase shift region whichis formed only in a vicinity of an edge portion of the opaque layer. Atleast a portion of the mask pattern is made up of the opaque layer andthe phase shift region, so that a phase shift occurs between a phase ofthe light transmitted through the phase shift region and a phase of thelight transmitted through a portion of the mask having no phase shiftregion.

A further object of the present invention is to provide a mask producingmethod comprising the steps of forming a resist pattern which is made ofa resist material on a transparent layer which is transparent withrespect to a light which is used for an exposure, forming a phase shiftlayer which transmits the light on the resist pattern to a predeterminedthickness, so that a phase shift occurs between a phase of the lighttransmitted through the phase shift layer and a phase of the lighttransmitted through a portion of the mask having no phase shift layer,and removing the resist pattern.

Another object of the present invention is to provide a mask producingmethod comprising the steps of forming a stopper layer on a transparentlayer which is transparent with respect to a light which is used for anexposure, forming a phase shift layer which transmits the light on thestopper layer to a predetermined thickness, so that a phase shift occursbetween a phase of the light transmitted through the phase shift layerand a phase of the light transmitted through a portion of the maskhaving no phase shift layer, forming a resist pattern which is made of aresist material on the phase shift layer, and etching the phase shiftlayer using the stopper layer as an etching stopper and the resistpattern as an etching mask.

Still another object of the present invention is to provide a maskproducing method comprising the steps of forming an opaque pattern layeron a transparent layer which is transparent with respect to a lightwhich is used for an exposure, forming a resist pattern which is made ofa resist material on the opaque pattern layer, forming a phase shiftlayer which transmits the light on the resist pattern layer to apredetermined thickness, so that a phase shift occurs between a phase ofthe light transmitted through the phase shift layer and a phase of thelight transmitted through a portion of the mask having no phase shiftlayer, and removing the resist pattern layer so that the phase shiftlayer remains on the transparent layer and the opaque pattern layer onlyin a vicinity of an edge portion of the opaque pattern layer in at leastone part of the mask.

A further object of the present invention is to provide a mask producingmethod comprising the steps of forming an opaque pattern layer on atransparent layer which is transparent with respect to a light which isused for an exposure, forming a stopper layer on the opaque patternlayer, forming a phase shift layer which transmits the light on theopaque pattern layer to a predetermined thickness, so that a phase shiftoccurs between a phase of the light transmitted through the phase shiftlayer and a phase of the light transmitted through a portion of the maskhaving no phase shift layer, forming a resist pattern which is made of aresist material on the phase shift layer, and etching the phase shiftlayer using the stopper layer as an etching stopper and the resistpattern as an etching mask so that the phase shift layer remains on thestopper layer only in a vicinity of an edge portion of the opaquepattern layer in at least one part of the mask.

Another object of the present invention is to provide a mask producingmethod comprising the steps of forming an opaque pattern layer on atransparent layer which is transparent with respect to a light which isused for an exposure, forming a resist pattern which is made of a resistmaterial on the opaque pattern layer, and forming a phase shift regionin a vicinity of an edge portion of the opaque pattern layer in at leastone part of the mask by etching the transparent layer to a predeterminedthickness using the resist pattern as an etching mask, so that a phaseshift occurs between a phase of the light transmitted through the phaseshift region and a phase of the light transmitted through a portion ofthe mask having no phase shift region.

Still another object of the present invention is to provide a maskproducing method comprising the steps of forming a phase shift layer,forming an opaque pattern layer on the phase shift layer which istransparent with respect to a light which is used for an exposure,etching the phase shift layer to a predetermined thickness in a vicinityof an edge portion of the opaque pattern layer using the opaque patternas an etching mask to form a phase shift region in the vicinity of theedge portion of the opaque pattern layer, forming a resist pattern whichis made of a resist material on a selected portion of the opaque patternlayer, and etching a remaining portion of the opaque pattern, so that aphase shift occurs between a phase of the light transmitted through thephase shift region and a phase of the light transmitted through aportion of the mask having no phase shift region.

A further object of the present invention is to provide a mask producingmethod comprising the steps of forming an opaque pattern layer on atransparent layer which is transparent with respect to a light which isused for an exposure, etching the transparent layer to a predeterminedthickness in a vicinity of an edge portion of the opaque pattern layerusing the opaque pattern as an etching mask to form a phase shift regionin the vicinity of the edge portion of the opaque pattern layer, forminga resist pattern which is made of a resist material on a selectedportion of the opaque pattern layer, and etching a remaining portion ofthe opaque pattern layer, so that a phase shift occurs between a phaseof the light transmitted through the phase shift region and a phase ofthe light transmitted through a portion of the mask having no phaseshift region.

Another object of the present invention is to provide a mask producingmethod comprising the steps of forming an opaque pattern layer on a topof a transparent layer which is transparent with respect to a lightwhich is used for an exposure, etching the transparent layer using theopaque pattern as an etching mask, forming a resist layer which is madeof a resist material on the opaque pattern layer, exposing the resistlayer from a bottom of the transparent layer so as to develop the resistlayer into a resist pattern, removing a portion of the opaque patternlayer by a side etching, and forming a phase shift region by removingthe resist pattern in a vicinity of an edge portion of the opaquepattern layer, so that a phase shift occurs between a phase of the lighttransmitted through the phase shift region and a phase of the lighttransmitted through a portion of the mask having no phase shift region.

Still another object of the present invention is to provide a maskproducing method comprising the steps of forming an opaque pattern layeron a top of a transparent layer which is transparent with respect to alight which is used for an exposure, etching the transparent layer usingthe opaque pattern as an etching mask, forming a resist layer which ismade of a resist material on the opaque pattern layer, exposing theresist layer from a bottom of the transparent layer so as to develop theresist layer into a resist pattern and to expose a portion of the opaquepattern layer, and forming a phase shift region by removing the exposedportion of the opaque pattern layer in a vicinity of an edge portion ofthe opaque pattern layer, so that a phase shift occurs between a phaseof the light transmitted through the phase shift region and a phase ofthe light transmitted through a portion of the mask having no phaseshift region.

A further object of the present invention is to provide a patternforming method comprising the steps of illuminating a mask by a lightfrom a light source, imaging the light transmitted through the mask ontoa photoresist layer which is formed on a wafer by use of a lens systemso as to develop a pattern on the photoresist layer, and developing thepattern on the photoresist layer, where the mask comprises a transparentlayer which is transparent with respect to the light from the lightsource and a mask pattern layer which is formed on the transparentlayer, and at least a portion of the mask pattern layer is made upsolely of a phase shift layer for transmitting the light, so that aninterference occurs between the light transmitted through the phaseshift layer and the light transmitted through a portion of the maskhaving no phase shift layer.

Another object of the present invention is to provide a pattern formingmethod comprising the steps of illuminating a mask by a light from alight source, imaging the light transmitted through the mask onto aphotoresist layer which is formed on a wafer by use of a lens system soas to develop a pattern on the photoresist layer, and developing thepattern on the photoresist layer, where the mask has a mask pattern andcomprises a transparent layer which is transparent with respect to thelight, an opaque layer which is formed on the transparent layer, and aphase shift region which is formed in a vicinity of an edge portion ofthe opaque layer, and at least a portion of the mask pattern is made upof the opaque layer and the phase shift region, so that an interferenceoccurs between the light transmitted through the phase shift region andthe light transmitted through a portion of the mask having no phaseshift region.

Other objects and further features of the present invention will beapparent from the following detailed description when read inconjunction with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a cross sectional view showing a conventional mask providedwith an opaque layer;

FIG. 2 generally shows an optical system which forms a pattern using themask shown in FIG. 1;

FIGS. 3A through 3D are diagrams for explaining a conventional patternforming method;

FIGS. 4A through 4D show light intensity distributions of the prior art;

FIGS. 5A through 5D are cross sectional views for explaining a method ofproducing a conventional mask which has a phase shift layer and uses anegative resist;

FIG. 6 shows a phase profile of the light transmitted through the maskshown in FIG. 5D;

FIG. 7 is a cross sectional view for explaining a method of producing aconventional mask which has a phase shift layer and uses a positiveresist;

FIG. 8 shows a phase profile of the light transmitted through the maskshown in FIG. 7;

FIG. 9 is a cross sectional view showing a first embodiment of a maskaccording to the present invention;

FIGS. 10A through 10C are diagrams for explaining the light intensity atone edge portion of a phase shift layer;

FIGS. 11A through 11C are diagrams for explaining the light intensity attwo edge portions of the phase shift layer;

FIG. 12 generally shows an optical system which is used for the exposureof the first embodiment of the mask;

FIG. 13 is a diagram for explaining a first embodiment of a patternforming method according to the present invention;

FIGS. 14A through 14E are cross sectional views for explaining a firstembodiment of a mask producing method according to the presentinvention;

FIGS. 15A through 15F are cross sectional views for explaining a secondembodiment of the mask producing method according to the presentinvention;

FIG. 16 is a plan view showing a mask pattern;

FIGS. 17A and 17B are diagrams for explaining the light intensitydistribution in FIG. 16;

FIGS. 18A through 18C are diagrams for explaining a second embodiment ofthe pattern forming method according to the present invention;

FIG. 19 is a cross sectional view showing a second embodiment of themask according to the present invention;

FIGS. 20A and 20B are diagrams for explaining the light intensity at oneedge portion of the phase shift layer;

FIGS. 21A and 21B are diagrams for explaining the light intensity at twoedge portions of the phase shift layer;

FIG. 22 is a diagram for explaining a third embodiment of the patternforming method according to the present invention;

FIGS. 23A and 23B are diagrams for explaining a pattern in FIG. 19;

FIGS. 24A and 24B are diagrams for explaining the light intensitydistribution in FIG. 23;

FIG. 25 is a diagram for explaining the second embodiment of the maskapplied to the line-and-space pattern;

FIGS. 26A through 26D are diagrams for explaining the light intensitydistribution for different dimension conversion quantities and widths ofthe phase shift layer;

FIG. 27 is a diagram for explaining the conventional light intensitydistribution in correspondence with FIG. 25;

FIGS. 28A through 28F are diagrams for explaining the light intensitydistribution for the case where a KrF excimer laser is used in FIG. 25;

FIGS. 29A and 29B are diagrams showing a portion of FIG. 19;

FIGS. 30A and 30B are diagrams for explaining the light intensitydistribution in FIG. 23;

FIGS. 31A and 31B are diagrams for explaining the formation of a blackpattern on a white background;

FIGS. 32A and 32B are diagrams for explaining the light intensitydistribution in FIGS. 31A and 31B;

FIGS. 33A and 33B are diagrams for explaining the case where the secondembodiment of the mask is applied to the formation of an integratedcircuit pattern;

FIGS. 34A through 34C are diagrams showing a portion of the patternshown in FIG. 19;

FIGS. 35A and 35B are diagrams for explaining the light intensitydistribution when a pattern width h=0.3 μm;

FIGS. 36 through 41 respectively are diagrams for explaining the lightintensity distributions when the pattern width h=0.4, 0.5, 0.6, 0.7, 0.8and 1.0 μm;

FIGS. 42A and 42B are diagrams for explaining the case where the secondembodiment of the mask is applied to the formation of a contact holepattern;

FIGS. 43A and 43B are diagrams for explaining the light intensitydistribution in FIGS. 42A and 42B;

FIGS. 44A through 44F are cross sectional views for explaining a thirdembodiment of the mask producing method according to the presentinvention;

FIGS. 45A through 45E are cross sectional views for explaining a fourthembodiment of the mask producing method according to the presentinvention;

FIGS. 46A through 46E are cross sectional views for explaining a fifthembodiment of the mask producing method according to the presentinvention;

FIG. 47 shows a phase profile of the light transmitted through the maskshown in FIG. 46E;

FIGS. 48A through 48C are cross sectional views for explaining a sixthembodiment of the mask producing method according to the presentinvention;

FIG. 49 shows a phase profile of the light transmitted through the maskshown in FIG. 48C;

FIGS. 50A through 50H are diagrams for explaining a seventh embodimentof the mask producing method according to the present invention;

FIGS. 51A through 51G are diagrams for explaining an eighth embodimentof the mask producing method according to the present invention;

FIGS. 52A through 52F are diagrams for explaining a ninth embodiment ofthe mask producing method according to the present invention;

FIGS. 53A through 53F are diagrams for explaining a tenth embodiment ofthe mask producing method according to the present invention;

FIGS. 54A through 54F are diagrams for explaining an eleventh embodimentof the mask producing method according to the present invention;

FIGS. 55A through 55C are diagrams for explaining the pattern formingmethod using the mask;

FIGS. 56A and 56B show light intensity distributions for explaining theimaging using the edge portion of the phase shift layer;

FIGS. 57A and 57B are diagrams for explaining the formation of a loopshaped pattern in a fourth embodiment of the pattern forming methodaccording to the present invention;

FIGS. 58A and 58B are diagrams for explaining the formation of an openshaped pattern in the fourth embodiment of the pattern forming methodaccording to the present invention;

FIGS. 59A and 59B are diagrams for explaining the formation of a pointpattern in the fourth embodiment of the pattern forming method accordingto the present invention;

FIGS. 60A through 60C are diagrams for explaining the formation of anintersecting line pattern in the fourth embodiment of the patternforming method according to the present invention;

FIGS. 61A and 61B are diagrams for explaining the formation of aT-shaped pattern in the fourth embodiment of the pattern forming methodaccording to the present invention;

FIGS. 62A through 62D are diagrams for explaining the formation of aninterconnection pattern in the fourth embodiment of the pattern formingmethod according to the present invention;

FIGS. 63A through 63C are diagrams for explaining the intersectingpattern;

FIGS. 64A through 64C are diagrams for explaining the interconnectionpattern;

FIGS. 65A through 65C are diagrams for explaining the interconnectionpattern;

FIGS. 66A through 66C are diagrams for explaining the oval pattern;

FIGS. 67A and 67B are diagrams for explaining the line-and-spacepattern;

FIGS. 68A and 68B are diagrams for explaining a mixed pattern of varioussizes;

FIG. 69 is a cross sectional view showing a sixth embodiment of the maskaccording to the present invention;

FIGS. 70A through 70E, FIGS. 71A through 71D and FIGS. 72A through 72Dare diagrams for explaining the light intensity distribution for phaseshift layers having different thicknesses;

FIG. 73 is a diagram for explaining a fifth embodiment of the patternforming method according to the present invention; and

FIGS. 74A, 74B, 75A, 75B, 76A, 76B, 77A and 77B are diagrams showingexamples of the patterns formed according to the present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

A description will be given of a first embodiment of a mask according tothe present invention, by referring to FIG. 9. In FIG. 9, a mask 1comprises a transparent substrate 2 which is transparent with respect toa light L which is used for the exposure, and a mask pattern layer 5which is formed on the transparent substrate 2. At least one portion ofthe mask pattern layer 5 is made up solely of a phase shift layer 3a.The light L can be transmitted through the phase shift layer 3a.

The phase of the light which is transmitted through the phase shiftlayer 3a is shifted from the phase of the light which is transmittedthrough only the transparent substrate 2. Accordingly, an interferenceoccurs and the light intensity decreases at the boundary of the lightwhich is transmitted through only the transparent substrate 2 and thelight which is transmitted through the phase shift layer 3a. For thisreason, it is possible to form on a wafer (not shown) an interferencepattern which is smaller than the wavelength of the light L which isused for the exposure. In addition, it is possible to adjust the phaseshift quantity of the light and improve the resolution of the exposurepattern by adjusting the thickness of the mask pattern layer 5.

FIGS. 10A through 10C are diagrams for explaining the light intensity atone edge portion of the phase shift layer 3a of the mask 1. In FIG. 10A,the phase of the light transmitted through the transparent substrate 2and the phase shift layer 3a is shifted by approximately 180° withrespect to the light transmitted through only the transparent substrate2. Hence, an electrical vector E and a light intensity P of the lighttransmitted through the mask i respectively become as shown in FIGS. 10Band 10C. As may be seen from FIG. 10C, it is possible to expose a linepattern by using the light intensity change at the edge portion of thephase shift layer 3a.

FIGS. 11A through 11C are diagrams for explaining the light intensity attwo edge portions of the phase shift layer 3a of the mask 1. In FIGS.11A through 11C, those parts which are the same as those correspondingparts in FIGS. 10A through 10C are designated by the same referencenumerals, and a description thereof will be omitted. In this case, thelight intensity P of the light transmitted through the mask 1 becomes asshown in FIG. 11C when a width W of the phase shift layer 3a issufficiently small. Thus, it is possible to control the width of theline pattern by controlling the width W.

FIG. 12 generally shows an optical system which is used for the exposureusing the mask 1. For example, a light source 6 is made up of a mercurylamp and the mercury lamp is provided with a filter (not shown) forpassing only the i-ray which has the wavelength of 365 nm. The lightfrom the light source 6 passes through an illumination lens system 8 andreaches the mask 1 as the light L. The mask 1 is positioned at the focaldistance of the lens system 8. The partial coherency σ of the light L is0.5 in this embodiment, but the partial coherency σ may be selectedwithin a range such that 0.3≦σ≦0.7.

The light transmitted through the mask 1 is imaged on a wafer 11 whichis coated with a photoresist layer 10 via an imaging lens system 9. Theimaging lens system 9 includes a 1/5 reduction lens and has a numericalaperture NA of 0.5. The wafer 11 is maintained flat by a chuck (notshown).

Next, a description will be given of a first embodiment of a patternforming method according to the present invention, by referring to FIG.13 which shows the exposure state in FIG. 12. The light which passesthrough the illumination lens system 8 shown in FIG. 12 is transmittedthrough the mask 1, and the light which is transmitted through the mask1 can be divided into a light 7a which is transmitted through the maskpattern layer 5 of the mask and a light 7b which is transmitted througha portion of the mask 1 not provided with the mask pattern layer 5. Thephase of the light 7a differs by 180° with respect to the phase of thelight 7b.

The lights 7a and 7b is imaged on the photoresist layer 10 on the wafer11 via the imaging lens system 9. A sharp light intensity change occursat a portion which corresponds to the edge portion of the phase shiftlayer 3a due to the interference. Hence, it is possible to form on thewafer 11 a pattern which is smaller than the wavelength of the lightused for the exposure.

Next, a description will be given of first and second embodiments of amask producing method according to the present invention, by referringto FIGS. 14 and 15. In FIGS. 14 and 15, those parts which areessentially the same as those corresponding parts in FIG. 9 aredesignated by the same reference numerals.

In FIG. 14A, the transparent substrate 2 is made of a material such asquartz and glass which transmits the i-ray. A resist material 3 isformed on the transparent substrate 2 as shown in FIG. 14B. A resistsuch as an electron beam (EB) resist and a photoresist is used as theresist material 3. When the EB resist is used as the resist material 3,a resist pattern 4 shown in FIG. 14C is formed by drawing by theelectron beam and carrying out a developing process. Then, a siliconoxide which constitutes the phase shift layer 3a is sputtered on thesurface of the resist pattern 4 as shown in FIG. 14D to a thickness of0.388 μm. Thereafter, the EB resist is removed by using a resist removalagent so that the mask pattern layer 5 of the phase shift layer (siliconoxide layer) 3a is formed on the transparent substrate 2 as shown inFIG. 14E.

On the other hand, in the embodiment shown in FIGS. 15A through 15F, thetransparent substrate 2 shown in FIG. 15A is the same as that shown inFIG. 14A but an aluminum oxide thin film 12 is formed on the transparentsubstrate 2 as shown in FIG. 15B. As shown in FIG. 15C, a silicon oxidewhich constitutes the phase shift layer 3a is sputtered on the thin film12 to a thickness of 0.388 μm. Furthermore, the resist material 3 isformed on the phase shift layer 3a as shown in FIG. 15D. A resistpattern 4 shown in FIG. 15E is formed by drawing by the electron beamand carrying out a developing process. Thereafter, a plasma etching or areactive ion etching (RIE) is carried out on the phase shift layer 3a byusing a CF₄ gas. The resist material 3 is removed by the ashing of theoxygen plasma, and the mask pattern 5 of the phase shift layer (siliconoxide layer) 3a is formed as shown in FIG. 15F. Because the thin film 12is not etched by the CF₄ gas, the thin film 12 acts as an etchingstopper. Thus, it is possible to obtain the mask pattern 5 which is madeup of the phase shift layer 3a accurately having the thickness of 0.388μm.

The thickness of the phase shift layer 3a is set to 0.388 μm for thepurpose of shifting the phase of the i-ray by 180°, that is, invertingthe phase. The phase shift quantity and the thickness of the phase shiftlayer 3a can be described by the following general formula (2), where ndenotes the refractive index of the phase shift layer 3a, λ denotes thewavelength of the light L used, S denotes the (phase shift quantity)/2π(or 1/2 when inverting the phase), and t denotes the thickness of thephase shift layer 3a.

    (n·t/λ)-(t/λ)=S                     (2)

In the embodiments shown in FIGS. 14 and 15, the refractive index n ofthe phase shift layer 3a is 1.47, the wavelength λ of the light L usedis 0.365 nm, and the phase shift quantity S is 1/2. Hence, the generalformula (2) can be rewritten as the following formula (3).

    (1.47t/0.365)-(t//0.365)= 1/2                              (3)

Therefore, the thickness t of the phase shift layer 3a is calculated as0.388 μm from the formula (3). The phase shift quantity S is set to 1/2(180° ) in these embodiments because 180° is an optimum phase shiftquantity for forming the pattern by the interference.

FIG. 16 shows a plan view of the mask pattern 5 of the mask 1 which isproduced in the above described manner. When forming a line-and-spacepattern with the resolution limit of 0.35 μm by use of a 1/5 recutionlens system, a light intensity shown in FIG. 17B can be obtained on thewafer 11 when a width b is set to 0.15 μm (reticle size of 0.75 μm) anda width a of a region 13 having no phase shift layer 3a is set to 0.55μm (reticle size of 2.75 μm).

A comparison will now be made of FIGS. 17B and 17A. For the sake ofconvenience, it is assumed that the opaque layer 451 shown in FIG. 1 ismade of Cr and has a thickness in the range of 50 to 80 nm and a widthof 0.35 μm (reticle size of 1.75 μm) for the conventional method. Inaddition, it is assumed that the interval of the opaque pattern is setto 0.35 μm (reticle size of 1.75 μm) for the conventional method. Asshown in FIG. 17A, the light intensity is approximately 50% according tothe conventional method, and it is evident that the contrast is poor. Inthis state, it is impossible to form a pattern on the photoresist layer455 on the wafer 454 in FIG. 2. But on the other hand, in the abovedescribed embodiments, the light intensity is approximately 80% on thewafer 11 as shown in FIG. 17B. Accordingly, the light intensity isgreatly improved. In addition, the contrast (resolution) is greatlyimproved because there is no change at the dark portion.

Next, a description will be given of a second embodiment of the patternforming method according to the present invention, by referring to FIG.18. FIGS. 18A and 18B respectively are a plan view and a cross sectionalview showing the mask 1 which is for forming a pattern on the wafer,where large and fine patterns coexist in the pattern. An opaque layer 14which is made of a Cr layer having a thickness of 50 to 80 nm isindicated by a hatched region. In addition, an exposed portion of thetransparent substrate 2 is indicated by a white background. The exposedportion is divided into a large pattern region 15 and a fine patternregion 16. The phase shift layer 3a is formed within the fine patternregion 16. The large pattern region 15 is patterned by the conventionalmethod, while the fine pattern region 16 is formed by use of the phaseshift layer 3a. Large patterns and fine patterns usually coexist in theintegrated circuit. Hence, this embodiment is especially suited forpatterning the integrated circuit. FIG. 18C shows a pattern which isimaged on the wafer 11 by using the mask shown in FIGS. 18A and 18B.According to this embodiment, it is possible to form the pattern in asmall number of steps even when the pattern includes fine patterns, andit is possible to sufficiently improve the resolution.

Next, a description will be given of a second embodiment of the maskaccording to the present invention, by referring to FIG. 19. A mask 1Ashown in FIG. 19 comprises the transparent substrate 2, an opaque layer14 which is formed on the transparent substrate 2, and the phase shiftlayer 3a which is formed at the edge portion of the opaque layer 14. Forexample, the opaque layer 14 is made of Cr and is formed to a thicknessof 50 to 80 nm. For the sake of convenience, it is assumed that a finepattern is isolated on the left part of the transparent substrate 2 inFIG. 19 and fine patterns are located adjacent to each other on theright part of the transparent substrate 2. In addition, a pattern 3b forforming a fine pattern is made up solely of the phase shift layer 3a.This pattern 3b is transferred onto the wafer 11 by the interferencecaused by the phase shifter, as will be described later in thespecification. Distances a, b and c shown in FIG. 19 will also bedescribed later in the specification.

FIGS. 20A and 20B are diagrams for explaining the light intensity at theedge portion of the phase shift layer 3a of the mask 1A. In FIG. 20A,the phase of the light transmitted through the transparent substrate 2and the phase shift layer 3a is shifted by approximately 180° withrespect to the light transmitted through only the transparent substrate2. Hence, the light intensity P of the light transmitted through themask 1A becomes as shown in FIG. 20B. As may be seen from FIG. 20B, itis possible to expose a line pattern by using the light intensity changeat the edge portion of the phase shift layer 3a.

FIGS. 21A and 21B are diagrams for explaining the light intensity at twoedge portions of the phase shift layer 3a of the mask 1A. In FIGS. 21Aand 21B, those parts which are the game as those corresponding parts inFIGS. 20A and 20B are designated by the same reference numerals, and adescription thereof will be omitted. In this case, the light intensity Pof the light transmitted through the mask 1A becomes as shown in FIG.21B. Thus, it is possible to control the width of the line pattern bycontrolling the width W.

Next, a description will be given of a third embodiment of the patternforming method according to the present invention, by referring to FIG.22 which generally shows an optical system which uses the mask 1A forthe exposure. In FIG. 22, those parts which are essentially the same asthose corresponding parts in FIG. 13 are designated by the samereference numerals, and a description thereof will be omitted.

In FIG. 22, the light L which originates from the light source 6 andreaches the mask 1A via the illumination lens system 8 cannot betransmitted through the opaque layer 14. The phase of the light 7atransmitted through the phase shift layer 3a is shifted and is invertedwith respect to the phase of the light 7b which is transmitted throughonly the transparent substrate 2. The lights 7a and 7b are imaged on thephotoresist layer 10 on the wafer 11 via the imaging lens system 9. Inthis case, the lights 7a and 7b mutually interfere and a sharp lightintensity change occurs. Accordingly, it is possible to improve thecontrast between the portion provided with no opaque layer 14 and theportion provided with the opaque layer 14. The conditions of the opticalsystem shown in FIG. 22 are the same as those of the optical systemshown in FIG. 13. Thus, the thickness of the phase shift layer 3a is setto 0.388 μm and the phase shift quantity is set to 180° based on theformula (3).

Next, a description will be given of the effects obtained by forming thephase shift layer 3a at the edge portion of the opaque layer 14, byreferring to FIGS. 23 through 26.

FIG. 23A shows a case where no phase shift layer 3a is provided at theedge portion of the opaque layer 14, and FIG. 23B shows a case where thephase shift layer 3a is provided at the edge portion of the opaque layer3a with a width b of 0.15 μm. FIGS. 24A and 24B respectively show thelight intensity distributions for the cases shown in FIGS. 23A and 23B.In the case shown in FIG. 24B, it can be seen that the light intensitydistribution changes sharply at the portion of the phase shift layer 3ain a vicinity of 0.0 μm on the x-axis. In other words, it can be seenfrom FIG. 24B that the capability of forming the fine pattern is largein this case. In this case, the boundary of the phase shift layer 3a andthe exposed portion of the transparent substrate 2 is 0.0 μm on thex-axis. Thus, when the boundary of the opaque layer 14 and the phaseshift layer 3a is assumed to be -0.15 μm on the x-axis, the boundarybetween the black and white portions of the pattern becomes +0.1 μm fromFIG. 16B.

Therefore, when the light used for the exposure is the i-ray having thewavelength of 365 nm and the numerical aperture NA of the lens system is0.5, it is possible to form the resist pattern to the designed dimensionwhen the exposed portion of the transparent substrate 2 is made 0.2 μm(0.1 μm on each side) larger than the designed dimension and the phaseshift layer 3a is formed with a width of 0.15 μm on the periphery of theexposed portion of the transparent substrate 2. In the case where themask 1A is a reticle which has the pattern with a dimension five timesthat of the resist pattern to be exposed on the wafer 11, it is possibleto form the resist pattern to the designed dimension when the exposedportion of the transparent substrate 2 is made 1.0 μm (0.5 μm on eachside) larger than the designed dimension and the phase shift layer 3a isformed with a width of 0.75 μm on the periphery of the exposed portionof the transparent substrate 2. In the description given hereunder, itis assumed for the sake of convenience that the reticle is madeaccording to the same rule.

The above rule must be changed when the wavelength λ of the light or thenumerical aperture NA of the lens system changes. In other words, it isnecessary to change the dimension conversion quantity of the exposedportion of the transparent substrate 2 and the width of the phase shiftlayer 3a formed in the periphery of the edge portion of the opaque layer14. For example, when a KrF laser is used as the light source whichemits a light having a wave length of 248 nm and the lens system usedhas a numerical aperture of 0.48, it is possible to obtain an optimumeffect when the exposed portion of the transparent substrate 2 is made0.13 μm (0.065 μm on each side) larger than the designed dimension andthe phase shift layer 3a at the periphery of the edge portion of theopaque layer 14 is formed with a width of 0.06 μm. In the case where themask 1A is a reticle which has the pattern with a dimension five timesthat of the resist pattern to be exposed on the wafer 11, it is possibleto obtain the optimum effect when the exposed portion of the transparentsubstrate 2 is made 0.65 μm (0.325 μm on each side) larger than thedesigned dimension and the phase shift layer 3a is formed with a widthof 0.30 μm on the periphery of the exposed portion of the transparentsubstrate 2. When the white patterns close upon each other, the regionof the opaque layer 14 disappears between the white patterns and thewhite patterns are separated solely by the phase shift layer 3a.

It is known from experience that the resolution is improved by 20% whenthe phase shift method is employed. Hence, the dimension conversionquantity at the exposed portion of the transparent substrate 2 and thewidth of the phase shift layer 3a can be obtained from experience. Thatis, when the i-ray is used for the exposure and the numerical apertureNA of the lens system is 0.5, the resolution limit for the case where nophase shift is made can be described by 0.6×(λ/NA) and the resolutionlimit for the case where the phase shift is made can be described by0.6×(λ/NA)×0.8. Therefore, the width of the phase shift layer 3a can becalculated as 0.35 μm when numerical values are substituted.

FIG. 25 shows a case where this embodiment is applied to the so-calledline-and-space pattern. In FIG. 25, the phase shift layer 3a is formedat the edge portion of the opaque layer 14, and the phase shift layer 3ais also formed between the four spaces in which the transparentsubstrate 2 is exposed.

FIGS. 26A through 26D show the light intensity distributions of theline-and-space pattern with the resolution limit of 0.35 μm when thephase shift is made when the dimension conversion quantity a at theexposed portion of the transparent substrate 2 and the width b of thephase shift layer 3a are varied. FIG. 26A shows the light intensitydistribution for the case where a=0.45 μm and b=0.25 μm. FIG. 26B showsthe light intensity distribution for the case where a=0.50 μm and b=0.20μm. FIG. 26C shows the light intensity distribution for the case wherea=0.55 μm and b=0.15 μm. FIG. 26D shows the light intensity distributionfor the case where a=0.60 μm and b=0.10 μm. It is necessary thata+b=0.70 μm in order that the line-and-space pattern is formed. As maybe seen from FIGS. 26A through 26D, the peak of the light intensitydecreases when the width b of the phase shift layer 3a is large. Inaddition, when the width b of the phase shift layer 3a is small, thelight intensity at the space portion becomes large and the contrastdecreases. An optimum contrast is obtained when b=0.15 μm, and a=0.55 μmin this case. The boundary between the phase shift layer 3a and theexposed portion of the transparent substrate 2 is shifted so as toenlarge the exposed portion of the transparent substrate 2 by 0.10 μm oneach side from the designed dimension, and this case corresponds to theedge portion of the large pattern described above. The width b of thephase shift layer 3a must be set within 30 to 60% of the resolutionlimit, that is, within the range of 0.144 to 0.228 μm.

A high contrast can be obtained especially when the width b of the phaseshift layer 3a is set within 40 to 50% of the resolution limit.

FIG. 27 shows the light intensity distribution of the conventional maskshown in FIG. 1 for the case where the width of the exposed portion ofthe transparent substrate 452 is 0.35 μm and the width of the opaquelayer 451 is 0.35 μm. In the case where the mask shown in FIG. 27 is areticle which has the pattern with a dimension five times that of theresist pattern to be exposed on the wafer, FIG. 27 corresponds to thelight intensity distribution of the conventional reticle for the casewhere the width of the exposed portion of the transparent substrate 452is 1.75 and the width of the opaque layer 451 is 1.75 μm. In FIG. 27,the light intensity is approximately 55% while the light intensity isapproximately 80% in FIG. 26C. Hence, it is readily seen that thecontrast is greatly improved in this embodiment.

When the width b of the phase shift layer 3a is 30 to 60% of theresolution limit, this means that the phase shift pattern should besmaller than the designed pattern by 20 to 35% (one side) of theresolution limit. When the i-ray is used and the numerical aperture NAof the lens system is 0.5, the phase shift pattern is 0.070 to 0.123 μm(one side) smaller than the designed pattern. Furthermore, it is knownfrom experience that the width of the phase shift pattern for the casewhere the pattern of the phase shift layer is arranged between theopaque layer and the exposed portion of the transparent substrate isdesirably 1.0 to 1.5 times the reduced width which is 20 to 35% (oneside) of the resolution limit. This width is 0.070 to 0.185 μm when thei-ray is used and the numerical aperture NA of the lens system is 0.5.

When the KrF excimer laser is used to emit a light having a wavelengthof 248 nm and the lens system used has a numerical aperture NA of 0.48,the resolution limit is 0.25 μm when the phase shift is made. FIGS. 28Bthrough 28F show the light intensity distributions of for this case whenthe dimension conversion quantity a at the exposed portion of thetransparent substrate 2 and the width b of the phase shift layer 3a arevaried. FIG. 28A shows for comparison purposes the light intensitydistribution for the case where no phase shift is made. FIG. 28B showsthe light intensity distribution for the case where a=0.35 μm and b=0.15μm. FIG. 28C shows the light intensity distribution for the case wherea=0.36 μm and b=0.14 μm. FIG. 28D shows the light intensity distributionfor the case where a=0.38 μm and b=0.12 μm. FIG. 28E shows the lightintensity distribution for the case where a=0.40 μm and b=0.10 μm. FIG.28F shows the light intensity distribution for the case where a=0.42 μmand b=0.08 μm. An optimum contrast is obtained when b=0.12 μm as may beseen from FIG. 28D. The width b of the phase shift layer 3a must be setwithin 30 to 60% of the resolution limit.

FIG. 29A shows a portion of the mask 1A shown in FIG. 19. In FIG. 29A,the phase shift layer 3a is formed at both edge portions of the opaquelayer 14, and the transparent substrate 2 is exposed between the phaseshift layers 3a. On the other hand, FIG. 29B shows a portion of theconventional mask 450 shown in FIG. 1 for comparison purposes. In FIG.29B, the mask pattern made solely of the opaque layer 451 is formed onthe transparent substrate 452.

When comparing FIGS. 29A and 29B, it is assumed that the width a of theexposed portion of the transparent substrate 2 shown in FIG. 29A is 0.55μm (reticle size of 2.75 μm), the width b of the phase shift layer 3a is0.15 μm (reticle size of 1.75 μm), and the width d of the exposedportion of the transparent substrate 452 shown in FIG. 29B is 0.35 μm(reticle size of 1.75 μm). FIGS. 30A and 30B respectively show the lightintensity distributions for the cases shown in FIGS. 29A and 29B. As maybe seen from FIG. 30A, the light intensity is approximately 100% in thisembodiment while the light intensity is approximately 65% in theconventional case, and it can be seen that the contrast is greatlyimproved according to this embodiment. Therefore, it is possible toobtain satisfactory results when the exposed portion of the transparentsubstrate 2 is formed larger than the designed pattern which is to beformed on the wafer and the phase shift layer 3a is formed on theperipheral portion of the exposed portion of the transparent substrate2.

Next, a description will be given of a case where a black pattern isformed on a white background, by referring to FIGS. 31A and 31B. FIG.31A shows a mask for forming a black pattern of 0.35 μm using the phaseshift. According to the rule described above, the mask pattern is madeup solely of the phase shift layer 3a which has a width c of 0.15 μm(reticle size of 0.75 μm). In this case, the light intensity of thepattern imaged on the wafer becomes as shown in FIG. 32A. On the otherhand, FIG. 31B shows a mask for forming the black pattern of 0.35 μmwithout the use of the phase shift. The mask shown in FIG. 31B is madeup of the opaque layer 14 having a width of 0.35 μm (reticle size of1.75 μm). In this case, the light intensity of the pattern imaged on thewafer becomes as shown in FIG. 32B. As may be seen by comparing FIGS.32A and 32B, there is a sharp decrease in the light intensity when thephase shift is used, and the resolution of the black pattern isimproved.

FIGS. 33A and 33B show the case where the this embodiment is applied tothe formation of the integrated circuit pattern. FIG. 33A is a plan viewshowing a portion of the mask 1A, and FIG. 33B is a plan view showing apattern which is imaged on the wafer 11. In FIG. 33A, the phase shiftlayer 3a is formed at the edge portion of the opaque layer 14 and at theexposed portion of the transparent substrate 2 where the fine patternsare adjacent to each other. In other words, in FIG. 33A there areprovided an isolated region 15 and a pattern adjacent region 16.

FIG. 34A shows a pattern which is formed on the wafer 11 using the mask1A, and a description will now be given of the interference caused bythe mask 1A. In FIG. 34A, it is assumed that a pattern width g is fixedto 0.35 μm and a pattern width h is varied to take the values 0.35, 0.4,0.5, 0.6, 0.7, 0.8 and 1.0 μm. In this case, according to the ruledescribed above, the mask pattern for the case where the patterns withthe pattern width h of 0.35, 0.4 and 0.5 μm are adjacent becomes asshown in FIG. 34B wherein the mask pattern is made up solely of thephase shift layer 3a and the white patterns are isolated. In addition,the mask pattern for the case where the patterns with the pattern widthh of 0.6, 0.7, 0.8 and 1.0 μm are isolated from each other to a certainextent becomes as shown in FIG. 34C wherein a region of the opaque layer14 exists between the white patterns.

For the sake of convenience, it is assumed that the pattern width h onthe wafer 11 is 0.35 μm. FIG. 35A shows the light intensity distributionfor the mask shown in FIG. 34B when the mask dimension a is 0.55 μm(reticle size of 2.75 μm), b is 0.15 μm (reticle size of 0.75 μm) and cis 0.15 μm (reticle size of 0.75 μm). On the other hand, FIG. 35B showsthe light intensity distribution for the conventional mask shown in FIG.21B when the mask dimension e is 0.35 μm (reticle size of 1.75 μm) and fis 0.35 μm (reticle size of 1.75 μm). When FIGS. 35A and 35B arecompared, it can be seen that the light intensity is approximately 85%according to this embodiment while the light intensity is approximately55% in the conventional case, and the contrast is improved according tothis embodiment.

FIG. 36 shows the light intensity distribution when the pattern width his 0.4 μm, FIG. 37 shows the light intensity distribution when thepattern width h is 0.5 μm, FIG. 38 shows the light intensitydistribution when the pattern width h is 0.6 μm, FIG. 39 shows the lightintensity distribution when the pattern width h is 0.7 μm, FIG. 40 showsthe light intensity distribution when the pattern width h is 0.8 μm, andFIG. 41 shows the light intensity distribution when the pattern width his 1.0 μm. As is also clear from FIGS. 36 through 41, the lightintensity is large and the contrast is improved according to thisembodiment.

The following Table shows the pattern dimension of the isolated pattern15 shown in FIG. 33A for various values of the pattern width h.

                  TABLE                                                           ______________________________________                                        h (μm)     Pattern Dimension (μm)                                       ______________________________________                                        0.35          0.35                                                            0.40          0.36                                                            0.50          0.35                                                            0.60          0.35                                                            0.70          0.35                                                            0.80          0.35                                                            1.00          0.35                                                            Isolated Pattern                                                                            0.35                                                            ______________________________________                                    

As may be seen from the Table, the change in the pattern dimension ofthe isolated pattern 15 caused by the interference is within ±0.01 μm.Accordingly, it is possible to control the pattern dimension. But inorder to more accurately control the pattern dimension, it is preferableto change the values of the pattern widths a, b, c and i in FIGS. 34Band 34C.

Next, a description will be given of the case where this embodiment isapplied to the formation of a contact hole pattern. FIG. 42A shows themask 1A for forming a contact hole of 0.35 μm. According to the ruledescribed above, the width a of the exposed portion of the transparentsubstrate 2 is 0.55 μm and the width b of the phase shift layer 3a is0.15 μm. FIG. 43A shows the light intensity distribution for this caseusing the mask 1A shown in FIG. 42A. On the other hand, FIG. 42B showsthe conventional mask for forming the contact hole of 0.35 μm withoutthe phase shift. In this case, the width l of the exposed portion of thetransparent substrate 2 is 0.35 μm. FIG. 43B shows the light intensitydistribution for this case using the mask shown in FIG. 42A. As may beseen from FIGS. 43A and 43B, the contrast of the contact hole can begreatly improved according to this embodiment.

Next, a description will be given of third and fourth embodiments of themask producing method according to the present invention, by referringto FIGS. 44A through 44F and FIGS. 45A through 45E. In FIGS. 44A through44F and FIGS. 45A through 45E, those parts which are essentially thesame as those corresponding parts in FIGS. 14 and 15 are designated bythe same reference numerals, and a description thereof will be omitted.

The transparent substrate 2 shown in FIG. 44A is made of quartz, forexample. The opaque layer 14 is formed on the transparent substrate 2 asshown in FIG. 44B. For example, the opaque layer 14 is made of Cr and isformed to a thickness of 50 to 80 nm. A resist material 3 such as an EBresist is formed on the opaque layer 14. The resist material issubjected to a usual mask production process (not shown) including theelectron beam drawing, developing, etching and resist removal steps.Thereafter, a pattern 20 of the opaque layer 14 is formed as shown inFIG. 44C. The resist material 3 which is an EB resist is formed on thepattern 20, and the resist pattern 4 shown in FIG. 44D is formed byelectron beam drawing and developing steps. Furthermore, the phase shiftlayer 3s is formed on the resist pattern 4 as shown in FIG. 44E. Forexample, the phase shift layer 3a is made of a silicon oxide and has athickness of 0.388 μm. Next, the resist material 3 is removed by use ofa removal agent and the mask pattern layer 5 of the opaque layer 14 andthe phase shift layer 3a is formed as shown in FIG. 44F.

In the embodiment shown in FIG. 45, the pattern 20 of the opaque layer14 is formed similarly to the steps shown in FIGS. 44A through 44C. Forexample, the opaque layer 14 is made of Cr and has a thickness of 50 to80 nm. A stopper layer 21 is formed on the pattern 20 as shown in FIG.45B. The stopper layer 21 acts as a stopper when making a plasma etchingof an oxide layer on the pattern 20.

For example, the stopper layer 21 is formed on the pattern 20 bysputtering a thin film of aluminum oxide which acts as a stopper for theplasma etching which uses CF₄. The phase shift layer 3a is formed on thestopper layer 21 as shown in FIG. 45C to a desired thickness. Forexample, the phase shift layer 3a is made of silicon oxide. The desiredthickness of the phase shift layer 3a is set to 0.388 μm in thisembodiment by use of the formula (3) so as to make a phase shift of180°. The resist material 3 is formed on the phase shift layer 3a, andthe resist pattern 4 is formed as shown in FIG. 45D by electron beamdrawing and developing steps. Then, the exposed phase shift layer 3a isetched by the CF₄ plasma and the resist material 3 is removed so as toform the mask pattern layer 5 which comprises the phase shift layer 3aas shown in FIG. 45E.

Next, a description will be given of a fifth embodiment of the maskproducing method according to the present invention, by referring toFIGS. 46A through 46E. This embodiment of the mask producing methodproduces a third embodiment of the mask according to the presentinvention shown in FIG. 46E is produced. The third embodiment of themask has a designed pattern having a width of 1 μm, for example, and hasa structure for shifting the phase of the light wherein a regioncorresponding to the phase shift pattern of the glass substrate islocated at a position deeper than that of the designed pattern.

In FIG. 46A, a glass substrate 31 is made of quartz and has a thicknessof 2 to 3 mm. An opaque layer 32 is formed on the glass substrate 31.For example, the opaque layer 32 is made of Cr and is formed on theglass substrate 31 by a sputtering CR to a thickness of 500 to 1000 Å.In addition, an aperture pattern 33 is formed in the opaque layer 32 bya normal lithography using the electron beam exposure. The aperturepattern 33 has a width of 0.85 μm in the case of the designed patternhaving the width of 0.35 μm.

Next, in FIG. 46B, a conductor layer 34 is formed on the glass substrate31 for preventing charge up at the time of the electron beam exposure.For example, the conductor layer 34 is made of MoSi₂ and is sputtered onthe glass substrate 31 to a thickness of 200 to 300 Å. Thereafter, anegative EB resist layer 35 is formed on the glass substrate 31 andprebaked. In addition, the pattern having the width of 0.55 μm issubjected to an electron beam exposure approximately above the centralportion of the aperture pattern 33 in the opaque layer 32. An exposedportion of the EB resist layer 35 is denoted by 35A.

Then, as shown in FIG. 46C, a normal developing is made to selectivelyleave the exposed resist layer 35A on the designed pattern formingregion. In addition, an etching process is carried out using a gasmixture of CCl₄ and O₂ so as to selectively remove the exposed conductorlayer 34 by using the exposed resist layer 35A as a mask.

In FIG. 46D, the exposed resist layer 35A and the opaque layer 32 areused as masks and an RIE which uses a gas mixture of CF₄ and O₂ iscarried out on the top surface of the glass substrate 31 which isexposed in correspondence with phase shift patterns. Phase shiftpatterns 37A and 37B are formed on respective sides of a pattern 36having a width of 0.55 μm on the glass substrate 31. The surface of thepattern 36 coincides with the top surface of the glass substrate 31. Thephase shift patterns 37A and 37B have a width of 0.15 μm and has a topsurface which is lower than the surface of the pattern 36. In order toshift the phase of the light which is transmitted through the phaseshift patterns 37A and 37B by 180° with respect to the light which istransmitted through the pattern 36, the phase shift patterns 37A and 37Bare etched to a depth D. When the i-ray having the wavelength of 365 nmis used for the exposure and the refractive index of glass making up theglass substrate 31 is 1.54, the depth D in this case is approximately0.36 μm from the formula (1).

Next, in FIG. 46E, the exposed resist layer 35A is removed by a normalashing process. In addition, the conductor layer 34 is removed by a dryetching processing using a gas mixture of CCl₄ and O₂. As a result, amask 1C is completed. This mask 1C is the third embodiment of the maskaccording to the present invention.

FIG. 47 shows a phase profile of the light transmitted through the mask1C in correspondence with the mask 1C shown in FIG. 46E. In FIG. 47,i_(a) denotes the light transmitted through the central portion andi_(b) denotes the light transmitted through the phase shift patterns 37Aand 37B.

According to the mask 1C, the phase shift pattern is made deep by usingthe negative resist at the time of patterning the phase shift pattern.However, when a positive resist is used the central portion is madedeeper than the phase shift pattern. A fourth embodiment of the maskaccording to the present invention has such a structure and may beproduced by steps similar to those shown in FIGS. 46A through 46E.

Next, a description will be given of a sixth embodiment of the maskproducing method according to the present invention, by referring toFIGS. 48A through 48C. In this embodiment, a mask 1D shown in FIG. 48Cis produced. In FIGS. 48A through 48C, those parts which are essentiallythe same as those corresponding parts in FIGS. 46A through 46E aredesignated by the same reference numerals, and a description thereofwill be omitted.

In FIG. 48A, the opaque layer 32 is formed on the glass substrate 31,and the aperture pattern 33 is formed in the opaque layer 32. Theaperture pattern 33 has a width of 0.85 μm. The conductor layer 34 forpreventing charge up is formed on the glass substrate 31, and a positiveEB resist layer 38 is thereafter formed on the glass substrate 31. Then,an electron beam exposure is made with respect to the designed patternwhich is located on the resist layer 38 at the central portion of theaperture pattern 33 in the opaque layer 32. An exposed region is denotedby 38A.

In FIG. 48B, the exposed region 38A of the resist layer 38 is removed bya developing process. A dry etching using a gas mixture of CCl₄ and O₂removes the conductor layer 34 exposed at an aperture 39 in the resistlayer 38. An RIE using a gas mixture of CF₄ and O₂ etches the topsurface of the glass substrate 31 which is exposed within the aperture39 similarly to the above described embodiment. The glass substrate 31is etched to the depth D which is approximately 0.36 μm.

In FIG. 48C, the resist layer 38 is removed by an ashing process, andthe conductor layer 34 is removed by a dry etching using a gas mixtureof CCl₄ and O₂. Accordingly, the mask 1D is completed.

FIG. 49 shows a phase profile of the light transmitted through the mask1D in correspondence with the mask 1D shown in FIG. 48C. In FIG. 49,i_(c) denotes the light transmitted through the central poriton andi_(d) denotes the light transmitted through the phase shift patterns 41Aand 41B.

Next, a description will be given of a seventh embodiment of the maskproducing method according to the present invention, by referring toFIGS. 50A through 50H. In FIG. 50A, a transparent substrate 51 is madeof glass, quartz or the like. A phase shift layer 52 is formed on thetransparent substrate 51 as shown in FIG. 50B. For example, the phaseshift layer 52 is made of silicon oxide and is formed by a chemicalvapor deposition (CVD), a sputtering, a spin-on-glass (SOG) techniqueand the like. In this embodiment, the phase shift layer 52 has athickness of 3900 Å so as to invert the phase of the light.

In FIG. 50C, an opaque layer 53 is formed on the phase shift layer 52.For example, the opaque layer 53 is made of Cr and has a thickness of700 to 1000 Å.

Next, an EB resist is formed on the opaque layer 53. Electron beamdrawing, developing and etching processes are carried out with respectto the opaque layer 53. As a result, a pattern 53a of the opaque layer53 is formed as shown in FIG. 50D.

The pattern 53a of the opaque layer 53 is used as a mask when etchingthe phase shift layer 52 as shown in FIG. 50E. In this embodiment, thephase shift layer 52 is an oxide layer and an RIE using a gas mixture ofCF₄ and CHF₃ is used for the etching.

Then, an EB resist layer 54 is formed as shown in FIG. 50F, and anunwanted portion 53b of the opaque layer 53 is exposed by electron beamdrawing and developing processes.

Finally, the EB resist layer 54 is removed and the unwanted portion 53bof the opaque layer 53 is removed by an etching so as to complete a mask1E. The mask 1E corresponds to a fifth embodiment of the mask accordingto the present invention. FIG. 50H shows a plan view of the mask 1E incorrespondence with FIG. 50G.

Next, a description will be given of an eighth embodiment of the maskproducing method according to the present invention, by referring toFIGS. 51A through 51G. In FIGS. 51A through 51G, those parts which areessentially the same as those corresponding parts in FIGS. 50A through50H are designated by the same reference numerals, and a descriptionthereof will be omitted. Steps shown in FIGS. 51A through 51Crespectively correspond to the steps shown in FIGS. 50A through 50C, butthe phase shift layer 52 is omitted in this embodiment.

In FIG. 51D, the pattern 53a of the opaque layer 53 is used as a maskwhen etching the transparent substrate 51. In this embodiment, an RIEusing a gas mixture of CF₄ and CHF₃ is used for the etching, and thetransparent substrate 51 is etched to a predetermined depth. Wheninverting the phase of the light, the predetermined depth is set to 3900Å.

Then, an EB resist layer 54 is formed as shown in FIG. 51E, and theunwanted portion 53b of the opaque layer 53 is exposed by electron beamdrawing and developing processes.

Finally, the EB resist layer 54 is removed and the unwanted portion 53bof the opaque layer 53 is removed by an etching so as to complete themask 1C shown in FIG. 51F. FIG. 51G shows a plan view of the mask 1C incorrespondence with FIG. 51F.

According to the seventh and eighth embodiments of the mask producingmethod, the opaque layer is used as a mask when etching the phase shiftlayer or the transparent substrate. Hence, it is possible to prevent apositioning error between the pattern of the opaque layer and the phaseshift pattern by a self-alignment.

Next, a description will be given of a ninth embodiment of the maskproducing method according to the present invention, by referring toFIGS. 52A through 52F. In FIGS. 52A through 52F, those parts which areessentially the same as those corresponding parts in FIGS. 51A through51G are designated by the same reference numerals, and a descriptionthereof will be omitted.

In FIG. 52A, the opaque layer 53 is formed on the transparent substrate51 and a pattern is formed by an electron beam drawing after forming anEB resist layer (not shown) on the opaque layer 53. This pattern isdeveloped and the opaque layer 53 is patterned by an etching. Forexample, the opaque layer 53 is made of Cr and is sputtered on thetransparent substrate 51. The EB resist layer is removed after thepatterning.

In FIG. 52B, the pattern 53a of the opaque layer 53 is used as a maskwhen etching the transparent substrate 51 to a predetermined depth. Theexposed transparent substrate 51 is etched by a parallel plate type RIEapparatus with a high-frequency excitation of 13.56 MHz. For example,the etching is carried out with a power of approximately 0.1 W/cm² at apressure of 0.1 to 0.05 Torr using a gas mixture of CF₄ or CHF₃ and O₂as the etchant gas. The etching depth is controlled to λ/2(n-1) bydetecting the depth using an end point detector. When λ=0.365 μm,λ/2(n-1)=0.39 μm.

In FIG. 52C, a positive resist layer 55 (for example, OFPR-800) isformed on the transparent substrate 51 and the pattern 53c to athickness of approximately 1.0 μm after the etching of the transparentsubstrate 51 and prebaked at a temperature of 100° C.

In FIG. 52D, a light of the resist exposure region such as amonochromatic light of 40 to 60 mJ/cm² and a wavelength of 436 nmilluminates the transparent substrate 51 in its entirety from a backsurface of the transparent substrate 51 so as to expose only the resistlayer 55 on the transparent substrate 51. Thereafter, the transparentsubstrate 51 is submerged in a solution with 2.38% of TMAH for 40seconds, and a developing process is carried out so that only the resistlayer 55 remains on the pattern 53c. Then, rinse and dry processes arecarried out.

After the resist pattern is formed, a side etching is carried out toremove a portion of the pattern 53c. The side etching quantity iscontrolled by the etching time so that approximately 0.4 to 0.8 μm ofthe opaque pattern 53 is removed on each side as shown in FIG. 52E.

Finally, the resist layer 55 remaining on the opaque layer 53 is removedby an O₂ ashing in FIG. 52F, and a mask which is substantially the sameas the mask 1D shown in FIG. 48C is completed.

Next, a description will be given of a tenth embodiment of the maskproducing method according to the present invention, by referring toFIGS. 53A through 53F. In FIGS. 53A through 53F, those parts which areessentially the same as those corresponding parts in FIGS. 52A through52F are designated by the same reference numerals, and a descriptionthereof will be omitted.

Steps shown in FIGS. 53A through 53C respectively correspond to thesteps shown in FIGS. 52A through 52C. However, in FIG. 53B, thetransparent substrate 51 is etched λ/2(n-1). When λ=0.365 μm,λ/2(n-1)=0.39 μm.

In FIG. 53D, the light of the resist exposure region illuminates thetransparent substrate 51 in its entirety from the back surface of thetransparent substrate 51 so as to expose only the resist layer 55 on thetransparent substrate 51. Thereafter, a developing process is carriedout so that only the resist layer 55 remains on the pattern 53c. Then,in this embodiment, an overexposure is made, an overdeveloping is made,or an ashing is made for a short time after the developing and drying soas to partially expose the opaque layer 53.

Next, as shown in FIG. 53E, the exposed opaque layer 53 is etched usingthe resist layer 55 as an etching mask.

In FIG. 53F, the remaining resist layer 55 on the opaque layer 53 isremoved by an O₂ ashing. As a result, a mask which is substantially thesame as the mask 1D is completed.

Next, a description will be given of an eleventh embodiment of the maskproducing method according to the present invention, by referring toFIGS. 54A through 54E. In FIGS. 54A through 54E, those parts which areessentially the same as those corresponding parts in FIGS. 52A through52F are designated by the same reference numerals, and a descriptionthereof will be omitted.

A step shown in FIG. 54A may be the same as the step shown in FIG. 52A.But in this embodiment, the resist layer 55 is formed on the transparentsubstrate 51 as shown in FIG. 54B without etching the transparentsubstrate 51.

In FIG. 54C, only the resist layer 55 on the transparent substrate 51 isremoved similarly to the step shown in FIG. 52D. Thereafter, the resistlayer 55 is used as an etching mask to etch the transparent substrate 51λ/2(n-1). When λ=0.365 μm, λ/2(n-1)=0.39 μm.

In FIG. 54D, a portion of the pattern 53c of the opaque layer 53 isremoved by a side etching. A step shown in FIG. 54E may be the same asthe step shown in FIG. 52F.

According to the embodiments described in conjunction with FIGS. 52Athrough 54E, there is no need to match the positions of the pattern ofthe opaque layer and the phase shift pattern. Thus, it is possible toproduce a mask having a pattern with a high accuracy and no positioningerror. In addition, the adhesion between the phase shift layer and thesubstrate, the cleaning and the like may be affected depending on thematerial used for the phase shift layer when the phase shift layer andthe substrate are independent. However, according to these embodiments,a portion of the transparent substrate itself is used as the phase shiftlayer or phase shift region, and the material of the phase shift layerdoes not become a problem. Accordingly, there is no need to take intoconsideration the stability and durability of the material used for thephase shift layer, and it is possible to easily form fine patternsexceeding the resolution limit of the exposure apparatus.

Next, a detailed description will be given of the pattern forming methodaccording to the present invention. FIGS. 55A through 55C are diagramsfor explaining the pattern forming method using the edge portion of thephase shift layer of the mask, and corresponds to FIGS. 10A through 10C.In FIGS. 55A through 55C, those parts which are essentially the same asthose corresponding parts in FIG. 12 are designated by the samereference numerals, and a description thereof will be omitted.

The phase shift layer having a refractive index n and a thickness t isformed on the transparent substrate 2 so as to constitute the mask 1.The pattern of the mask 1 is imaged on the photoresist layer 10 on thesemiconductor wafer 11 via the imaging lens system 9. The desiredpattern is imaged by utilizing the edge portion of the phase shift layer3a.

The light incident to the mask 1 is transmitted through the phase shiftlayer 3a. Since the refractive index n of the phase shift layer 3a isdifferent from that of air or vacuum, the phase of the transmitted lightis shifted. The phase shift quantity S is described by S=(n-1)t/λ or2π(n-1)t/λ in radians. For the sake of convenience, it is assumedhereunder that the phase shift quantity S is π so as to invert the phaseof the transmitted light.

As shown in FIG. 55B, the phase of the electrical vector E of the lighttransmitted through the mask 1 is inverted in correspondence with thepattern of the phase shift layer 3a. When the light having theelectrical vector distribution shown in FIG. 55B illuminates thephotoresist layer 10 and is absorbed thereby, the light intensitydistribution on the photoresist layer 10 becomes proportional to E².Hence, as shown in FIG. 55C, a black pattern PB having a narrow width isformed at the position where the phase transition occurs. In otherwords, when the sign of the electrical vector E changes in FIG. 55B,there is a point where the electrical vector E becomes zero. The lightintensity distribution is proportional to E² and a minimum value of thelight intensity P is zero at the point where the electrical vector E iszero. Accordingly, it is possible to obtain a clear black pattern.

Generally, when the phase shift layer 3a shifts the phase of thetransmitted light, it is possible to obtain a pattern in which the phaseshift is spatially distributed. When the phase shift is 180°, the lightintensity P at the position where the 180° phase shift occurs is alwayszero, and a time integral thereof is also zero. Hence, the lightintensity P at the edge portion of the phase shift layer 3a is zero.When the phase shift is other than 180°, the sign of the electricalvector E becomes the same or the opposite depending on the time, and thetime integral of the light intensity P does not become zero.

A description will now be given of an imaging using the edge portion ofthe phase shift layer, by referring to FIGS. 56A and 56B.

FIG. 56A shows the light intensity distribution for a case where thephase shift layer makes a 180° phase shift, that is, inverts the phaseof the transmitted light. In FIG. 56A, a region on the left of theabscissa 0.0 corresponds to a region in which the phase shift quantityis 180°, and this region is adjacent to an aperture portion on the rightwith the phase shift quantity of zero at the position 0.0. FIG. 56Ashows the light intensity distribution at one edge portion of the phaseshift layer. The light intensity decreases to approximately zero at thecentral portion of the pattern corresponding to the end of the phaseshift layer. By utilizing this imaging principle, it is possible toobtain a line pattern exceeding the conventional resolution limit. Inaddition, it is possible to obtain similar effects if the phasedifference at the edge portion of the phase shift layer is 180°±30°.

FIG. 56B shows the light intensity distribution for a case where thephase shift layer makes a 90° phase shift. When the phase shift quantityis 90°, the phase of the light transmitted through the phase shift layerand the phase of the light transmitted through the aperture portion maycoincide or be inverted. Hence, the minimum value of the light intensitydoes not become zero when the time integral thereof is taken. In FIG.56B, the minimum value of the light intensity is approximately 0.5 orgreater and is in the order of 1/2 the light intensity which isapproximately 1.0 at the uniform portion. When the developing level istaken less than this light intensity, it is possible to carry out thedeveloping in which the pattern shown is neglected. If the phasedifference at the edge portion of the phase shift layer is 90°±15°, thepattern is neglected and not formed under normal developing conditions(developing level of 35%). In addition, when the developing level istaken at an intermediate level between the minimum value and the uniformvalue of the light intensity, the pattern is developed at the centralportion thereof.

Next, a description will be given of a fourth embodiment of the patternforming method according to the present invention, by referring to FIGS.57A and 57B. FIGS. 57A and 57B are diagrams for explaining the formationof a loop shaped pattern according to this embodiment.

FIG. 57A shows a mask pattern. A mask 61 is formed on an aperture 60 inthe transparent substrate. The mask 61 has a phase shift quantitycorresponding to an optical path difference of the half-waves of theincoming light, and shifts the phase of the transmitted light by 180°.When such a mask pattern is imaged, the imaged pattern becomes as shownin FIG. 57B. The portion where only the aperture 60 or the phase shiftlayer 61 is imaged has a constant light intensity and is white. A blackpattern 62 is formed at a boundary portion of the aperture 60 and thephase shift layer 61. In other words, when the light transmitted throughthe aperture 60 and the light transmitted through the phase shift layer61 mix due to interference, mutually opposite phases cancel each otherand the light intensity becomes zero, thereby forming a black pattern.

When the phase shift layer 61 is made of a silicon oxide and the i-rayfrom the mercury lamp is the light which is used for the exposure, therefractive index of the phase shift layer 61 is approximately 1.47 whichhas a difference of approximately 0.47 with respect to the refractiveindex of air which is approximately 1.00. The thickness of the phaseshift layer 61 is approximately 0.388 μm. In order to cancel theamplitude of the mixed light due to the interference, the invertedphase, that is, 180° phase shift, is most effective. However, the phaseshift quantity is not limited to 180° and the light intensity can beeffectively reduced within the range of 180°±30%.

When the phase shift layer 61 having the phase shift quantity of 180° isformed above the aperture which makes no phase shift as in the case ofthe mask pattern shown in FIG. 57A, a black pattern is formed at theportion corresponding to the edge portion of the phase shift layer 61.In this case, the black pattern has a closed loop shape.

FIGS. 58A and 58B respectively show a mask pattern for forming an openshape (line segment) and an imaged pattern thereof.

In FIG. 58A, the phase shift layer 61 is formed above the aperture 60,and one side of the phase shift layer 61 is used for forming a linesegment. A phase shift layer 64 having a phase shift quantity of 90° isformed adjacent to the remaining (unwanted) sides of the phase shiftlayer 61. In other words, out of the four sides of the phase shift layer61 having the phase shift quantity of 180°, one side 61a forms aboundary with a region in which the phase shift quantity is zero, andthis side 61a forms a black pattern 65 in the imaged pattern as shown inFIG. 58B. The other sides 61b, 61c and 61d are adjacent to the phaseshift layer 64 which has the phase shift quantity of 90°. Hence, thephase shift when crossing these sides 61b, 61c and 61d is 90°, and thedecrease in the light intensity is small at the edge portion of thephase shift layer 61 corresponding to these sides 61b, 61c and 61d. Theperipheral sides of the phase shift layer 64 also have the phase shiftquantity of 90° and the decrease in the light intensity is similarlysmall. It is possible to leave only the black pattern 65 by adjustingthe developing level. In this case, a gray level is formed between thewhite and black levels, but it is of course possible to use two or morehalftones.

By using a plurality of kinds of phase shift layers having differentphase shift quantities, it is possible to form the pattern of an openshape such as a line segment.

FIGS. 59A and 59B respectively show a mask pattern for forming a dotpattern and an imaged pattern thereof.

In FIG. 59A, the phase shift layer 61 having the phase shift quantity of180° is formed above the aperture 60. The phase shift layer 64 which hasthe phase shift quantity of 90° surrounds the phase shift layer 61excluding one vertex portion thereof. In other words, the phase shiftlayer 61 is adjacent to the aperture 60 at only the one vertex portionof the phase shift layer 61. When such a mask pattern is imaged, theimaged pattern becomes as shown in FIG. 59B in which a black pattern 66is formed only at the portion where the phase shift layer 61 and theaperture 60 connect. Because the phase shift quantity is 90° at theboundary of the phase shift layers 61 and 64, the decrease in the lightintensity at this boundary is small compared to that at the portioncorresponding to the black pattern 66. In addition, the outer peripheralportion of the phase shift layer 64 also has the phase shift quantity of90°, and the decrease in the light intensity is similarly small. Forthis reason, it is possible to develop only the black pattern 66 as thepattern and treat the other patterns in the halftones as white patterns.

A description is given heretofore with respect to the cases where thepattern to be formed is a line segment pattern. But in the following, adescription will be given of the formation of a pattern which has anintersection.

FIGS. 60A and 60B show mask patterns for forming an intersectingpattern, and FIG. 60C shows an imaged pattern which is formed by themask patterns shown in FIGS. 60A and 60B.

FIG. 60A shows a first mask pattern. The surface of the mask pattern isdivided into four quadrants in correspondence with the regions which aredivided by the intersecting lines. The aperture 60 which makes no phaseshift is provided in the first quadrant. The phase shift layer 61 whichhas the phase shift quantity of 180° is provided in the two quadrantswhich are adjacent to the aperture 60. A phase shift layer 67 which hasa phase shift quantity of 360° is provided in the remaining quadrant. Inother words, the boundary between two quadrants has a phase shiftquantity of 180°.

FIG. 60B shows a second mask pattern. Similarly to the first maskpattern shown in FIG. 60A, the surface of the mask pattern shown in FIG.60B is divided into four quadrants. The aperture 60 which makes no phaseshift is provided in the first quadrant. The phase shift layer 61 whichhas the phase shift quantity of 180° is provided in the two quadrantsadjacent to the aperture 60. An aperture 68 which makes not phase shiftis provided in the remaining quadrant. In this case, the boundarybetween two quadrants also has a phase shift quantity of 180°.

When the mask patterns shown in FIGS. 60A and 60B are imaged, an imagedpattern shown in FIG. 60C is obtained. That is, a portion having auniform phase is imaged as a white pattern, and a portion having the180° phase shift is imaged as a black pattern 69.

The intersecting lines are not limited to straight lines and may becurved.

FIGS. 61A and 61B respectively show a mask pattern for forming aT-shaped pattern and an imaged pattern thereof.

In FIG. 61A, phase shift layers 61a and 61b having the phase shiftquantity of 180° are formed adjacent to the aperture 60 which makes nophase shift. In addition, the aperture 68 which makes no phase shift isformed above the phase shift layer 61a so as to form a boundary havingthe phase shift quantity of 180°. As a result, a black pattern shown inFIG. 61B is imaged when this mask pattern is imaged. Furthermore, sincea black pattern will be imaged if the phase shift layers 61b and 68 aredirectly adjacent to each other and form a boundary having the phaseshift quantity of 180°, a phase shift layer 71 which has a phase shiftquantity of 90° is formed between the phase shift layers 61b and 68. Inother words, the decrease in the light intensity is relatively small atthe boundary of the phase shift layer 71 because the phase shiftquantity is only 90°. By appropriately adjusting the developingthreshold value, such a decrease in the light intensity can be developedas a white pattern. As a result, a black pattern 72 shown in FIG. 61B isimaged.

In FIG. 61A, there is a gap between the aperture 68 and the phase shiftlayer 61b. However, such a gap may be reduced if needed.

In semiconductor devices, there is often a need to provide a wide regionfor making a contact at an intermediate part of an interconnectionpattern. FIGS. 62A through 62C show mask patterns for forming such aninterconnection pattern, and FIG. 63D shows an imaged pattern thereof.

In FIG. 62A, the aperture 60 which makes no phase shift and the phaseshift layer 61 which has the phase shift quantity of 180° are adjacentto each other and forms a linear boundary. A phase shift layer having aphase shift quantity of 180° is formed within the aperture 60 at thecentral portion of the boundary. An aperture 70 which makes no phaseshift is formed in the phase shift layer 61. The aperture 74 and thephase shift layer 75 both have a rectangular shape, and a boundarytherebetween is aligned to the linear boundary between the aperture 60and the phase shift layer 61. In FIG. 62A, all the boundaries indicatedby the solid line accompanies a 180° phase shift.

In FIG. 62B, the boundary which defines each region is formed similarlyto FIG. 62B. However, the aperture 74 shown in FIG. 62A is replaced by aphase shift layer 76 which has a phase shift quantity of 360°. As in thecase of the mask pattern shown in FIG. 62A, there is a 180° phase shiftbetween the upper and lower regions bounded by the straight horizontalboundary. In addition, a rectangular boundary having the phase shiftquantity of 180° is formed at the central portion of the mask pattern.

In FIG. 62C, the mask pattern is generally divided into two regions by astraight horizontal boundary. In the upper region, the aperture 60 whichmakes no phase shift is formed on the right and the phase shift layer 61which has the phase shift quantity of 180° is formed on the left. Theaperture 60 and the phase shift layer 61 are adjacent to each other fora limited length. In addition, a phase shift layer 77 which has anintermediate phase shift quantity of 90° is formed between the aperture60 and the phase shift layer 61. The lower region has a structuresimilar to that of the upper region. That is, in the lower region, theaperture 68 which makes no phase shift is formed on the left below thephase shift layer 61, and the phase shift layer 61 which has the phaseshift quantity of 180° is formed on the right below the aperture 60. Theaperture 68 and the phase shift layer 61 in the lower region areadjacent to each other for a limited length. In addition, the phaseshift layer 77 which has the intermediate phase shift quantity of 90° isformed between the aperture 68 and the phase shift layer 61 in the lowerregion. As a whole, a boundary which accompanies a 180° phase shift isformed along the straight horizontal line and also at a vertical linesegment portion at the central portion of the mask pattern.

FIG. 62D shows an imaged pattern which is obtained when the maskpatterns shown in FIGS. 62A through 62C are imaged. In this case, ablack pattern 78 which has a wide region at the central portion isimaged.

When making the length of the line finite in the mask patterns shown inFIGS. 60A, 60B, 61A, 62A, 62B and 62C, it is sufficient to form in anunwanted portion a region having an intermediate phase shift quantity of90° or the like.

The mask patterns for forming various imaged patterns are describedabove, and now, a description will be given of the light intensityprofiles which are obtained by numerical calculations for some of thedescribed mask patterns.

For the calculations, it is assumed for the sake of convenience that thelight used for the exposure has a wavelength of 365 nm, the imaging lenssystem has a numerical aperture NA of 0.50, and the light from theillumination lens system has a partial coherency σ of 0.50.

FIGS. 63A through 63C are diagrams for explaining the pattern ofintersecting lines.

FIG. 63A generally shows a pattern and a sampling region of theintersecting lines. The mask pattern shown in FIG. 60B is employed, andthe X, Y and Z coordinates shown on the right in FIG. 63A is used. Inaddition, a region indicated by a phantom line is regarded as thesampling region, and the light intensity within the sampling region iscalculated according to a mode. FIG. 63B is a graph showing the lightintensity profile within the sampling region shown in FIG. 63A as athree dimensional model. It may be seen from FIG. 63B that a deep valleyis formed at the boundary portion accompanying the 90° phase shift.Although FIG. 63B does not shows the light intensity profile for thelower half portion of the pattern shown in FIG. 63A, it is apparent tothose skilled in the art that the light intensity profile has astructure similar to that shown in FIG. 63B for the lower half portionof the pattern shown in FIG. 63A. FIG. 63C shows a projection of thelight intensity profile shown in FIG. 63B on an XY plane when the lightintensity profile is described by contour lines with respect to thelight intensity. In other words, regions with the minimum lightintensity are formed at a portion extending along the lower side in thedirection X and a portion extending along the central portion in thedirection Y. Portions where the light intensity gradually increases areformed adjacent to such regions with the minimum light intensity.

FIGS. 64A through 64C are diagrams for explaining the pattern ofinterconections.

FIG. 64A shows the mask pattern for the interconnection pattern shown inFIG. 62A, and a sampling region is indicated by a phantom line. The X, Yand Z coordinates shown on the right of FIG. 64A is used.

FIG. 64B shows a light intensity profile within the sampling regionshown in FIG. 64A. A narrow valley is formed along the direction X, andthe valley portion widens in a vicinity of a region in which X=0. FIG.64C shows a projection of the light intensity profile shown in FIG. 64Bon the XY plane when the light intensity profile is described by contourlines with respect to the light intensity. It may be seen from FIG. 64Cthat a valley portion is formed and the width of this valley portionwidens at the central portion.

FIGS. 65A through 65C are diagrams for explaining the pattern ofinterconnections.

FIG. 65A shows the mask pattern for the interconnection pattern shown inFIG. 62C, and a sampling region is indicated by a phantom line. On thehorizontal axis at the central portion of FIG. 65A there are formed theaperture 60 which makes no phase shift and the phase shift layer 61which has the phase shift quantity of 180°. The aperture 60 and thephase shift layer 61 are adjacent to each other in part and the phaseshift layer 77 which has the phase shift quantity of 90° is formedbetween the aperture 60 and the phase shift layer 61. In addition, theupper side of the aperture 60 makes an angle 81 to the horizontaldirection. The two sides of the phase shift layer (intermediate region)77 make an angle θ2. Further, the upper side of the phase shift layer 61makes an angle 83 to the horizontal direction. The phase shift layer 61which has the phase shift quantity of 180° is formed adjacent to andsymmetrically to the aperture 60 below the horizontal axis, and theaperture 68 which makes no phase shift is formed below the phase shiftlayer 61. In addition, another phase shift layer 77 is formed below thehorizontal axis at a position symmetrical to the phase shift layer 77above the horizontal axis. A narrow region at the central portion wherethe apertures 60 and 68 and the phase shift layer 61 meet has a widthW1. In this sample, θ1=θ2=θ3=60°, and W1=0.2 μm. FIGS. 65B and 65C showthe imaged pattern which is formed using such a mask pattern.

FIG. 65B shows a light intensity profile within the sampling regionshown in FIG. 65A. A deep valley is formed along the direction X, andthe valley extends in the direction Y at the central portion. Moreover,a shallow branching valley is formed as shown.

FIG. 65C shows a projection of the light intensity profile shown in FIG.65B on the XY plane when the light intensity profile is described bycontour lines with respect to the light intensity.

FIGS. 66A through 66C are diagrams for explaining a mask pattern havinga structure similar to the mask pattern shown in FIG. 59A.

In FIG. 66A, the aperture 60 which makes no phase shift is formed on theright and the phase shift layer 61 which has the phase shift quantity of180° and a triangular shape is formed on the left. The aperture 60 andthe phase shift layer 61 are adjacent to each other for a width W2. Inaddition, the phase shift layer 64 which has the phase shift quantity of90° is formed in each intermediate region between the aperture 60 andthe phase shift layer 61. The light intensity distribution is calculatedfor a sampling region indicated by a phantom line. For example, a gap of0.08 μm is set for the width W2, and two boundary lines which are formedby the phase shift layer 61 and the two phase shift layers 64 make anangle of 60°.

FIG. 66B shows the light intensity distribution within the samplingregion shown in FIG. 66A, and FIG. 66C shows a projection of the lightintensity profile shown in FIG. 66B on the XY plane when the lightintensity profile is described by contour lines with respect to thelight intensity. As may be seen from FIG. 66C, an oval region in whichthe light intensity is a minimum is formed in a vicinity of the position(0.0).

FIGS. 67A and 67B are diagrams for explaining a line-and-space pattern.

FIG. 67A shows a mask pattern for the line-and-space pattern. Theline-and-space pattern is formed by an aperture region 80 and a phaseshift region 81 which are alternately formed, where the aperture region80 makes no phase shift and the phase shift region 81 has a phase shiftquantity of 180°. For example, when the widths of the aperture region 80and the phase shift region 81 respectively are 0.5 μm and the lightintensity profile of the imaged pattern is calculated, the lightintensity profile shown in FIG. 67B is obtained. A minimum of the lightintensity is formed at a portion corresponding to the boundary betweenthe aperture region 80 and the phase shift region 81. When making thecalculation, it is assumed for the sake of convenience that the lightused for the exposure has the wavelength of 365 nm, the numericalaperture NA of the lens system is 0.53, and the partial coherency σ ofthe light is 0.5.

FIGS. 68A and 68B are diagrams for forming a pattern in which variouspattern sizes are mixed.

FIG. 68A shows a mask pattern. An opaque layer 85 for forming a largeblack pattern and a phase shift layer 86 are combined at the upper part.A phase shift layer 87 for forming the image of the phase shift layeritself is formed at the intermediate part. A phase shift layer 88 forforming an imaged pattern by the edge portion thereof is formed at thelower part. A phase shift layer 89 which has an intermediate phase shiftquantity for preventing the imaging of the unwanted sides is formed inthe periphery of the phase shift layer 88. For example, the phase shiftlayers 86, 87 and 88 have a phase shift quantity of 180°, and the phaseshift layer 89 has a phase shift quantity of 90°. When the lens systemused has a numerical aperture NA of 0.4 to 0.6 for the i-ray, thepattern of 0.5 μm or greater is formed by arranging the phase shiftregion 86 with the phase shift quantity of 90° around the opaque layer85 as shown in the upper part of FIG. 68A. When forming the blackpattern of 0.3 to 0.5 μm, the mask is formed solely by the pattern ofthe phase shift layer 87 having the phase shift quantity of 180°, asshown in the intermediate part of FIG. 68A. The pattern of 0.25 μm orless is formed by the boundary of the phase shift layer 88 which has thephase shift quantity of 180° and an aperture region 90 which makes nophase shift, as shown in the lower part of FIG. 68A. In this case, theside of the unwanted phase shift region 88 is surrounded by the phaseshift layer 89 which has the intermediate phase shift quantity so as notto be imaged by adjusting the developing level.

FIG. 68B shows the imaged pattern which is obtained by use of the maskpattern shown in FIG. 68A. A black pattern 91 which corresponds to themask which is made up of the opaque layer 85 and the phase shift region86 is formed at the upper part. A black pattern 92 which corresponds tothe phase shift layer 67 is formed at the intermediate part. A narrowblack pattern 93 which corresponds to the boundary between the phaseshift layer 88 and the aperture region 90 is formed at the lower part.

When producing an integrated circuit by the photolithography, thepattern of the reticle is imaged on the wafer using an imaging lenssystem. Accompanied by the increase in the integration density of theintegrated circuit, the numerical aperture NA of the imaging lens systemis increased so as to improve the resolution and cope with the fineintegrated circuit patterns. However, when the numerical aperture NA ismade large so as to improve the resolution, the focal point depth FDbecomes small according to the following formula (4), where K₂ denotes aprocess coefficient.

    FD=K.sub.2 (λ/NA.sub.2)                             (4)

Accordingly, it is impossible to accurately form the pattern withrespect to a concavoconvex surface.

Next, a description will be given of an embodiment which cansatisfactorily form a pattern even with respect to a concavoconvexsurface. FIG. 69 shows a sixth embodiment of the mask according to thepresent invention. In FIG. 69, those parts which are essentially thesame as those corresponding parts in FIG. 9 are designated by the samereference numerals, and a description thereof will be omitted. In thisembodiment, the phase shift layer 3a comprises a shifter portion 3a1having a thickness D1 and a shifter portion 3a2 having a thickness D1,where D2>D1. By providing such shifter portions which have differentthicknesses, it is possible to arbitrarily control the focal point ofthe image when making the exposure.

Next, a description will be given of a fifth embodiment of the patternforming method according to the present invention. In this embodiment,the thickness of the phase shift layer of the mask is made differentlocally where needed. For the sake of convenience, it is assumed thatthe mask 1A shown in FIG. 19 and the optical system shown in FIG. 22 isused to form the pattern, and a description will be given of therelationship of the thickness of the phase shift layer 3a and thedeviation (defocus quantity) of the focal position. The light used forthe exposure, the material used for the phase shift layer 3a and thelike are the same as those of the embodiment described above. When thethickness t of the phase shift layer 3a is calculated for obtaining eachphase shift quantity S of 180°, 120° and 240°, respectively, t=0.388,0.259 and 0.517 μm.

First, when t=0.388 μm and S=180°, the light intensity distributionbecomes as shown in FIG. 71C when the defocus quantity is zero. However,when the defocus quantities are set to +1.0 and +0.5 μm by taking as thepositive (+) direction the direction towards the wafer 11 along theoptical axis of the lens system 9, the light intensity distributionsbecome as shown in FIGS. 70A and 70B, respectively. On the other hand,when the defocus quantities are set to -0.5 and -1.0 μm, the lightintensity distributions become as shown in FIGS. 70D and 70E,respectively. As may be seen from FIGS. 70A through 70E, the lightintensity distribution changes in the same manner regardless of whetherthe focal point deviates in the positive direction or the negativedirection.

FIGS. 71A, 71B, 71C and 71D respectively show the light intensitydistributions which are obtained when t=0.259 μm, S=120° and the defocusquantities are to +1.0, +0.5, 0 and -0.5 μm. As may be seen from FIGS.71A through 71D, the contrast is a maximum when the defocus quantity is+0.5 μm.

FIGS. 72A, 72B, 72C and 72D respectively show the light intensitydistributions which are obtained when t=0.517 μm, S=240° and the defocusquantities are to +0.5, 0, -0.5 and -1.0 μm. As may be seen from FIGS.72A through 72D, the contrast is a maximum when the defocus quantity is-0.5 μm.

According to this embodiment, the focal point can be controlled withinthe range of 0.5 μm by appropriately determining the thickness t of thephase shift layer.

In the production process of the integrated circuit, a region of theintegrated circuit may have a height greater than that at other regions.For example, in the dynamic random access memory (DRAM), the memory cellpart is 0.5 to 1.0 μm higher than the other peripheral circuit partswhen stacked capacitors are used. In such a case, when an attempt ismade to form the pattern to the resolution limit by using a lens systemhaving a large numerical aperture NA, the focal point depth FD becomessmall and it is difficult to apply the pattern formation to theproduction of integrated circuits having concavoconvex surfaces.

However, according to this embodiment, it is possible to apply thepattern formation to the DRAM using stacked capacitors as shown in FIG.73. In FIG. 73, those parts which are essentially the same as thosecorresponding parts in FIGS. 22 and 69 are designated by the samereference numerals, and a description thereof will be omitted. In thiscase, the pattern formation of the lower portion of the integratedcircuit such as the peripheral circuit part of the DRAM is made by useof the thinner shifter portion 3a1 of the phase shift layer 3a.Accordingly, D1 is set smaller than 0.388 μm. For example, when thedifference in the heights of the cell part and the peripheral circuitpart of the DRAM is approximately 1.0 μm, D1 is set to 0.259 μm. On theother hand, the pattern formation of the high portion of the integratedcircuit such as the cell part of the DRAM is made by use of the thickershifter portion 3a2 of the phase shift layer 3a. Hence, D2 is setgreater than 0.388 μm. For example, D2 is set to 0.517 μm when the cellpart is approximately 1 μm higher than the peripheral circuit part ofthe DRAM.

By controlling the thickness of the phase shift layer depending on theconcavoconvex surface of the integrated circuit, it is possible to formthe pattern by matching the focal point to all surface portions of theintegrated circuit.

As described heretofore in conjunction with the various embodiments, thepresent invention forms the pattern by utilizing the phase shift layer.For this reason, it is possible to form an arbitrary fine pattern by useof a mask in which the phase shift layer is appropriately arranged.FIGS. 74A, 75A, 76A and 77A show examples of mask patterns and FIGS.74B, 75B, 76B and 77B respectively show patterns which can be formed byuse of the mask patterns shown in FIGS. 74A, 75A, 76A and 77A. In FIGS.74A through 77B, there are provided an opaque layer 90, a phase shiftlayer 91 and an aperture (window) 92.

In the described embodiments, the partial coherency σ is set to 0.5.However, the partial coherency σ is not limited to 0.5 and may be setwithin the range of 0.3≦σ≦0.7.

In addition, the light which is used for the exposure is not limited tothe i-ray. Moreover, the light use for the exposure may illuminate themask from the side of the transparent substrate provided within thephase shift layer or from the other side of the transparent substratenot provided with the phase shift layer. The materials used for thetransparent substrate and the phase shift layer are of course notlimited to those of the described embodiments. For example, thetransparent substrate may be made of any material as long as thetransparent substrate is transparent with respect to the light which isused for the exposure. Furthermore, the phase shift layer may be made ofa material selected from a group including SiO₂, Al₂ O₃ and MgF₂.

The masks described in the embodiments include reticles. Accordingly,the pattern forming method according to the present invention is notlimited to the application to the formation of patterns of semiconductordevices, and may of course be applied to the formation of patterns ofmasks and reticles.

Further, the present invention is not limited to these embodiments, butvarious variations and modifications may be made without departing fromthe scope of the present invention.

What is claimed is:
 1. A pattern forming method comprising the stepsof:illuminating a mask with a light from a light source whereby aportion of the light is transmitted through the mask; imaging the lighttransmitted through the mask onto a photoresist layer on a wafer using alens system so as to expose a pattern on the photoresist layer; anddeveloping the exposed pattern, said mask comprising a first layer whichis transparent with respect to said light and a mask pattern layer onsaid first layer, at least a portion of said mask pattern layercomprising a phase shift region which consists solely of a single layerphase shift material having a single thickness and adapted fortransmitting light and shifting the phase of the transmitted light,whereby the pattern on the photoresist layer is exposed by aninterference which occurs between light transmitted through said regionand light transmitted through other portions of the mask pattern layer.2. A pattern forming method as set forth in claim 1, wherein the phaseof the phase shift transmitted light is shifted within the range of from150° to 210° relative to the phase of exposure light which does notimpinge said region.
 3. A pattern forming method as set forth in claim1, wherein said phase shift region includes a straight edge disposedbetween said light portions for forming a line pattern.
 4. A patternforming method as set forth in claim 1, wherein said pattern comprisesone or more of said phase shift regions presenting a pair of straightedges disposed between said light portions for forming a pattern havingan arbitrary width.
 5. A pattern forming method as set forth in claim 1,wherein said pattern includes at least two phase shift regions and oneof said phase shift regions has a different thickness than another ofsaid regions.
 6. A pattern forming method as set forth in claim 5,wherein said imaging step comprises imaging of the pattern at differentfocal positions on said photoresist layer depending on a concave orconvex surface structure of said photoresist layer.
 7. A pattern formingmethod as set forth in claim 1, wherein said first layer is made ofquartz or glass.
 8. A pattern forming method as set forth in claim 1,wherein said phase shift region is made up of a material which comprisesSiO₂, Al₂ O₃ or MgF₂.
 9. A pattern forming method as set forth in claim1, wherein said mask includes an opaque mask layer covering portions ofthe first layer.
 10. A pattern forming method as set forth in claim 1,wherein said imaging step comprises the imaging on said photoresistlayer of a black pattern at a light intensity lower than that at otherportions of the photoresist layer by using phase interference generatedadjacent an edge portion of the phase shift region.
 11. A patternforming method as set forth in claim 1, wherein said imaging stepcomprises the imaging on said photoresist layer of a white pattern at alight intensity higher than that at other portions of the photoresistlayer by using phase interference generated adjacent an edge portion ofthe phase shift region.
 12. A pattern forming method as set forth inclaim 1, wherein the light utilized for illuminating the mask comprisesa partially coherent light having a partial coherency within the rangeof 0.3 to 0.7.
 13. A pattern forming method as set forth in claim 1,wherein the phase shift region comprises a portion of the first layerhaving a different thickness than other portions of the first layer. 14.A pattern forming method as set forth in claim 1, wherein said maskpattern includes mutually adjacent phase shift regions having mutuallydifferent phase shift quantities such that the light transmitted by onesuch region has a phase which differs by 90° to 180° from the phase ofthe light transmitted by the other such region.
 15. A pattern formingmethod as set forth in claim 1, wherein a black pattern is generated onsaid resist at the boundary between the adjacent phase shift regionswhen the same have a 180° phase difference, and wherein no pattern isgenerated on said resist at said boundary when the phase difference is90° or less.
 16. A pattern forming method as set forth in claim 1,wherein said mask pattern includes mutually adjacent phase shift regionshaving mutually different phase shift quantities such that the lighttransmitted by one such region has a phase which differs by 60° to 210°from the phase of the light transmitted by the other such region.
 17. Apattern forming method comprising the steps of:illuminating a mask witha light from a light source whereby a portion of the light istransmitted through the mask; imaging the light transmitted through themask onto a photoresist layer on a wafer using a lens system so as toexpose a pattern on the photoresist layer, said pattern being imaged atdifferent focal positions on said photoresist layer depending on aconcave or convex surface structure of the photoresist layer; anddeveloping the exposed pattern,said mask comprising a first layer whichis transparent with respect to said light, an opaque layer covering aportion of said first layer and having an edge portion, and a phaseshift region formed adjacent said edge portion of the opaque layer, saidopaque layer and said phase shift region presenting a mask pattern onsaid first layer, said phase shift region being adapted for transmittinglight and shifting the phase of the transmitted light, wherebyinterference occurs between light transmitted through said region andlight transmitted through other portions of the first layer.
 18. Apattern forming method as set forth in claim 17, wherein said firstlayer is made of quartz or glass.
 19. A pattern forming method as setforth in claim 17, wherein said mask includes a second opaque layercovering portions of the first layer.
 20. A method as set forth in claim17, wherein said imaging step comprises the imaging on said photoresistlayer of a black pattern at a light intensity lower than that at otherportions of the photoresist layer by using the interference generated inthe vicinity of the edge portion of the opaque layer.
 21. A method asset forth in claim 17, wherein said imaging step comprises the imagingon said photoresist layer of a white pattern at a light intensity higherthan that at other portions of the photoresist layer by using theinterference generated in the vicinity of the edge portion of the opaquelayer.
 22. A pattern forming method as set forth in claim 17, whereinthe light utilized for illuminating the mask comprises a partiallycoherent light having a partial coherency within the range of from 0.3to 0.7.
 23. A pattern forming method as set forth in claim 17,whereinthe phase shift region is bounded by the opaque layer and anexposed portion of the first layer, the mask and the photoresist layeron the wafer are aligned such that portions of the mask linearlycorrespond with portions of the photoresist layer, when lighttransmitted through the mask is imbed onto the photoresist layer, ablack and white boundary is created on the photoresist layerrepresenting darkened and illuminated portions of the photoresist layer,respectively, the boundary between the opaque layer and the phase shiftregion is located at a first distance along a predetermined directionrelative to the boundary between the phase shift region and the exposedportion of the first layer, and the black and white boundary on thephotoresist layer is located at a second distance along a directionopposite to the predetermined direction relative to the portion of thephotoresist layer corresponding to and aligned with the boundary betweenthe phase shift region and the exposed portion of the first layer.
 24. Apattern forming method comprising the steps of:illuminating a mask witha light from a light source whereby a portion of the light istransmitted through the mask; imaging the light transmitted through themask onto a photoresist layer on a wafer using a lens system so as toexpose a pattern on the layer; and developing the exposed pattern,saidmask comprising a first layer which is transparent with respect to saidlight, an opaque layer covering a portion of said first layer and havingan edge portion, and a phase shift region formed adjacent said edgeportion of the opaque layer, said opaque layer and said phase shiftregion presenting a first mask pattern on said first layer, said phaseshift region being adapted for transmitting light and shifting the phaseof the transmitted light, whereby interference occurs between lighttransmitted through said region and light transmitted through otherportions of the first layer, said mask including a second mask patternwhich is made up solely of a phase shift region.
 25. A pattern formingmethod comprising the steps of:illuminating a mask with a light from alight source whereby a portion of the light is transmitted through themask; imaging the light transmitted through the mask onto a photoresistlayer on a wafer using a lens system so as to expose a pattern on thelayer; and developing the exposed pattern,said mask comprising a firstlayer which is transparent with respect to said light, an opaque layercovering a portion of said first layer and having an edge portion, and aphase shift region formed adjacent said edge portion of the opaquelayer, said opaque layer and said phase shift region presenting a firstmask pattern on said first layer, said phase shift region being adaptedfor transmitting light and shifting the phase of the transmitted light,whereby interference occurs between light transmitted through saidregion and light transmitted through other portions of the first layer,said mask including a second mask pattern which is made up solely of anopaque layer and a third mask pattern which is made up solely of a phaseshift layer.
 26. A pattern forming method comprising the stepsof:illuminating a mask with a light from a light source whereby aportion of the light is transmitted through the mask; imaging the lighttransmitted through the mask onto a photoresist layer on a wafer using alens system so as to expose a pattern on the photoresist layer; anddeveloping the exposed pattern,said mask comprising a first layer whichis transparent with respect to said light, an opaque layer covering aportion of said first layer and having an edge portion, and a phaseshift region formed adjacent said edge portion of the opaque layer, saidopaque layer and said phase shift region presenting a mask pattern onsaid first layer, said phase shift region being adapted for transmittinglight and including first and second portions which have mutuallydifferent thicknesses and shifting the phase of the transmitted light,whereby interference occurs between light transmitted through saidregion and light transmitted through other portions of the first layer.27. A pattern forming method comprising the steps of:illuminating a maskwith a light from a light source whereby a portion of the light istransmitted through the mask; imaging the light transmitted through themask onto a photoresist layer on a wafer using a lens system so as toexpose a pattern on the photoresist layer; and developing the exposedpattern,said mask comprising a first layer which is transparent withrespect to said light and a mask pattern layer on said first layer, atleast a portion of said mask pattern layer comprising a phase shiftregion adapted for transmitting light and shifting the phase of thetransmitted light, whereby interference occurs between light transmittedthrough said region and light transmitted through other portions of themask pattern layer so that at least a portion of the pattern is formedby the interference between light transmitted through the phase shiftregion and light transmitted through the first layer without interactionof an opaque layer on the mask.